diff --git a/Network/Wai/Logger/Format.hs b/Network/Wai/Logger/Format.hs
--- a/Network/Wai/Logger/Format.hs
+++ b/Network/Wai/Logger/Format.hs
@@ -1,7 +1,9 @@
 {-# LANGUAGE OverloadedStrings #-}
 
 module Network.Wai.Logger.Format (
-    apacheFormat
+    IPAddrSource(..)
+  , apacheFormat
+  -- * Builder
   , apacheFormatBuilder
   ) where
 
@@ -17,10 +19,18 @@
 import Network.Wai.Logger.IO
 import Network.Wai.Logger.Utils
 
+{-| Source from which the IP source address of the client is obtained.
+-}
+data IPAddrSource =
+  -- | From the peer address of the HTTP connection.
+    FromSocket
+  -- | From X-Real-IP: or X-Forwarded-For: in the HTTP header.
+  | FromHeader
+
 -- | Apache style log format.
-apacheFormat :: ZonedDate -> Request -> Status -> Maybe Integer -> [LogStr]
-apacheFormat tmstr req st msize = [
-    LS addr
+apacheFormat :: IPAddrSource -> ZonedDate -> Request -> Status -> Maybe Integer -> [LogStr]
+apacheFormat ipsrc tmstr req st msize = [
+    getSourceIP ipsrc req
   , LB " - - ["
   , LB tmstr
   , LB "] \""
@@ -39,16 +49,13 @@
   , LB $ lookupRequestField' "user-agent" req
   , LB "\"\n"
   ]
-  where
-    addr = showSockAddr (remoteHost req)
 
-lookupRequestField' :: CI Ascii -> Request -> Ascii
-lookupRequestField' k req = fromMaybe "" . lookup k $ requestHeaders req
-
--- | Apache style log format with 'Builder'. This is experimental.
-apacheFormatBuilder :: ZonedDate -> Request -> Status -> Maybe Integer -> Builder
-apacheFormatBuilder tmstr req status msize =
-      st addr
+{-| Apache style log format with 'Builder'. This is experimental.
+    This would replace 'apacheFormat' someday.
+-}
+apacheFormatBuilder :: IPAddrSource -> ZonedDate -> Request -> Status -> Maybe Integer -> Builder
+apacheFormatBuilder ipsrc tmstr req status msize =
+      getSourceIP' ipsrc req
   +++ bs " - - ["
   +++ bs tmstr
   +++ bs "] \""
@@ -67,7 +74,25 @@
   +++ bs (lookupRequestField' "user-agent" req)
   +++ bs "\"\n"
   where
-    addr = showSockAddr (remoteHost req)
     st = fromString
     bs = fromByteString
     (+++) = mappend
+
+lookupRequestField' :: CI Ascii -> Request -> Ascii
+lookupRequestField' k req = fromMaybe "" . lookup k $ requestHeaders req
+
+getSourceIP :: IPAddrSource -> Request -> LogStr
+getSourceIP FromSocket req = LS $ showSockAddr (remoteHost req)
+getSourceIP FromHeader req = LB addr
+  where
+    maddr = lookup "x-real-ip" hdrs >> lookup "x-forwarded-for" hdrs
+    addr = fromMaybe "" maddr
+    hdrs = requestHeaders req
+
+getSourceIP' :: IPAddrSource -> Request -> Builder
+getSourceIP' FromSocket req = fromString . showSockAddr . remoteHost $ req
+getSourceIP' FromHeader req = fromByteString addr
+  where
+    maddr = lookup "x-real-ip" hdrs >> lookup "x-forwarded-for" hdrs
+    addr = fromMaybe "" maddr
+    hdrs = requestHeaders req
diff --git a/Network/Wai/Logger/IO.hs b/Network/Wai/Logger/IO.hs
--- a/Network/Wai/Logger/IO.hs
+++ b/Network/Wai/Logger/IO.hs
@@ -3,7 +3,9 @@
 
 module Network.Wai.Logger.IO (
     LogStr(..)
-  , hPutLogStr, hPutBuilder, initHandle
+  , hPutLogStr
+  -- * Builder
+  , initHandle, hPutBuilder
   ) where
 
 import Blaze.ByteString.Builder
@@ -23,10 +25,16 @@
 import GHC.Real
 import System.IO
 
--- | A date type to contain 'String' and 'ByteString'.
+{-| A date type to contain 'String' and 'ByteString'.
+    This data is exported so that format can be defined.
+    This would be replaced with 'Builder' someday.
+-}
 data LogStr = LS !String | LB !ByteString
 
--- | The 'hPut' function directory to copy a lift of 'LogStr' to the buffer.
+{-| The 'hPut' function to copy a list of 'LogStr' to the buffer
+    of 'Handle' directly.
+    If 'Handle' is associated with a file, 'AppendMode' must be used.
+-}
 hPutLogStr :: Handle -> [LogStr] -> IO ()
 hPutLogStr handle bss =
   wantWritableHandle "hPutLogStr" handle $ \h_ -> bufsWrite h_ bss
@@ -71,13 +79,16 @@
     poke dst (c2w x)
     copy' (dst `plusPtr` 1) xs
 
--- | Setting a proper buffering to 'Handle'.
+{-| Setting a proper buffering to 'Handle'.
+-}
 initHandle :: Handle -> IO ()
 initHandle hdl = hSetBuffering hdl (BlockBuffering (Just 4096))
 
 {-| The 'hPut' function directory to copy 'Builder' to the buffer.
+    If 'Handle' is associated with a file, 'AppendMode' must be used.
     The current implementation is inefficient at this moment.
     'initHandle' must be called once beforehand if this function is used.
+    This would replace 'hPutLogStr' someday.
 -}
 hPutBuilder :: Handle -> Builder -> IO ()
 hPutBuilder hdl = BS.hPut hdl . toByteString
diff --git a/Network/Wai/Logger/Prefork.hs b/Network/Wai/Logger/Prefork.hs
--- a/Network/Wai/Logger/Prefork.hs
+++ b/Network/Wai/Logger/Prefork.hs
@@ -4,6 +4,7 @@
   , logController
   , ApacheLogger
   , LogType(..)
+  , IPAddrSource(..)
   , FileLogSpec(..)
   , LogController
   ) where
@@ -11,6 +12,7 @@
 import Control.Concurrent
 import Control.Monad
 import Network.Wai.Logger.File
+import Network.Wai.Logger.Format
 import Network.Wai.Logger.Prefork.File
 import Network.Wai.Logger.Prefork.Stdout
 import Network.Wai.Logger.Prefork.Types
@@ -20,10 +22,10 @@
 logCheck LogStdout = return ()
 logCheck (LogFile spec) = check spec
 
-logInit :: LogType -> IO ApacheLogger
-logInit LogNone        = noLoggerInit
-logInit LogStdout      = stdoutLoggerInit
-logInit (LogFile spec) = fileLoggerInit spec
+logInit :: IPAddrSource -> LogType -> IO ApacheLogger
+logInit _ LogNone            = noLoggerInit
+logInit ipsrc LogStdout      = stdoutLoggerInit ipsrc
+logInit ipsrc (LogFile spec) = fileLoggerInit ipsrc spec
 
 noLoggerInit :: IO ApacheLogger
 noLoggerInit = return noLogger
diff --git a/Network/Wai/Logger/Prefork/File.hs b/Network/Wai/Logger/Prefork/File.hs
--- a/Network/Wai/Logger/Prefork/File.hs
+++ b/Network/Wai/Logger/Prefork/File.hs
@@ -30,14 +30,14 @@
 
 ----------------------------------------------------------------
 
-fileLoggerInit :: FileLogSpec -> IO ApacheLogger
-fileLoggerInit spec = do
+fileLoggerInit :: IPAddrSource -> FileLogSpec -> IO ApacheLogger
+fileLoggerInit ipsrc spec = do
     hdl <- open spec
     hdlref <- HandleRef <$> newIORef hdl
     forkIO $ fileFlusher hdlref
     dateref <- dateInit
     installHandler sigUSR1 (Catch $ reopen spec hdlref) Nothing
-    return $ fileLogger dateref hdlref
+    return $ fileLogger ipsrc dateref hdlref
 
 {-
  For BlockBuffering, hPut flushes the buffer before writing
@@ -61,11 +61,11 @@
 
 ----------------------------------------------------------------
 
-fileLogger :: DateRef -> HandleRef -> ApacheLogger
-fileLogger dateref hdlref req status msiz = do
+fileLogger :: IPAddrSource -> DateRef -> HandleRef -> ApacheLogger
+fileLogger ipsrc dateref hdlref req status msiz = do
     date <- getDate dateref
     hdl <- getHandle hdlref
-    hPutLogStr hdl $ apacheFormat date req status msiz
+    hPutLogStr hdl $ apacheFormat ipsrc date req status msiz
 
 fileFlusher :: HandleRef -> IO ()
 fileFlusher hdlref = forever $ do
diff --git a/Network/Wai/Logger/Prefork/Stdout.hs b/Network/Wai/Logger/Prefork/Stdout.hs
--- a/Network/Wai/Logger/Prefork/Stdout.hs
+++ b/Network/Wai/Logger/Prefork/Stdout.hs
@@ -8,13 +8,15 @@
 import Network.Wai.Logger.Prefork.Types
 import qualified Data.ByteString as BS
 
-stdoutLoggerInit :: IO ApacheLogger
-stdoutLoggerInit = stdoutLogger <$> dateInit
+stdoutLoggerInit :: IPAddrSource -> IO ApacheLogger
+stdoutLoggerInit ipsrc = stdoutLogger ipsrc <$> dateInit
 
-stdoutLogger :: DateRef -> ApacheLogger
-stdoutLogger dateref req status msiz = do
+stdoutLogger :: IPAddrSource -> DateRef -> ApacheLogger
+stdoutLogger ipsrc dateref req status msiz = do
     date <- getDate dateref
-    BS.putStr $ BS.concat $ map toBS $ apacheFormat date req status msiz
+    putToStdout $ logmsg date
   where
+    putToStdout = BS.putStr . BS.concat . map toBS
+    logmsg date = apacheFormat ipsrc date req status msiz
     toBS (LS s) = pack s
     toBS (LB s) = s
diff --git a/wai-logger.cabal b/wai-logger.cabal
--- a/wai-logger.cabal
+++ b/wai-logger.cabal
@@ -1,5 +1,5 @@
 Name:                   wai-logger
-Version:                0.0.1
+Version:                0.0.2
 Author:                 Kazu Yamamoto <kazu@iij.ad.jp>
 Maintainer:             Kazu Yamamoto <kazu@iij.ad.jp>
 License:                BSD3