diff --git a/Network/Wai/Middleware/Push/Referer.hs b/Network/Wai/Middleware/Push/Referer.hs
--- a/Network/Wai/Middleware/Push/Referer.hs
+++ b/Network/Wai/Middleware/Push/Referer.hs
@@ -13,11 +13,7 @@
 import Data.ByteString (ByteString)
 import qualified Data.ByteString as BS
 import Data.ByteString.Internal (ByteString(..), memchr)
-import Data.Map (Map)
-import qualified Data.Map.Strict as M
 import Data.Maybe (isNothing)
-import Data.Set (Set)
-import qualified Data.Set as S
 import Data.Word (Word8)
 import Data.Word8
 import Foreign.ForeignPtr (withForeignPtr, ForeignPtr)
@@ -29,6 +25,8 @@
 import Network.Wai.Internal (Response(..))
 import System.IO.Unsafe (unsafePerformIO)
 
+import qualified Network.Wai.Middleware.Push.Referer.LimitMultiMap as M
+
 -- $setup
 -- >>> :set -XOverloadedStrings
 
@@ -39,18 +37,18 @@
 --   this function should return 'Just'.
 --   If 'Nothing' is returned,
 --   the middleware learns nothing.
-type MakePushPromise = URLPath  -- ^ path in referer
-                    -> URLPath  -- ^ path to be pushed
-                    -> FilePath -- ^ file to be pushed
+type MakePushPromise = URLPath  -- ^ path in referer  (key: /index.html)
+                    -> URLPath  -- ^ path to be pushed (value: /style.css)
+                    -> FilePath -- ^ file to be pushed (file_path/style.css)
                     -> IO (Maybe PushPromise)
 
 -- | Type for URL path.
 type URLPath = ByteString
 
-type Cache = Map URLPath (Set PushPromise)
+type Cache = M.LimitMultiMap URLPath PushPromise
 
 emptyCache :: Cache
-emptyCache = M.empty
+emptyCache = M.empty 20 20 -- FIXME hard-coding
 
 cacheReaper :: Reaper Cache (URLPath,PushPromise)
 cacheReaper = unsafePerformIO $ mkReaper settings
@@ -59,30 +57,30 @@
 settings :: ReaperSettings Cache (URLPath,PushPromise)
 settings = defaultReaperSettings {
       reaperAction = \_ -> return (\_ -> emptyCache)
-    , reaperCons   = insert
-    , reaperNull   = M.null
+    , reaperCons   = M.insert
+    , reaperNull   = M.isEmpty
     , reaperEmpty  = emptyCache
 --  , reaperDelay  = 30000000 -- FIXME hard-coding
     }
 
-insert :: (URLPath,PushPromise) -> Cache -> Cache
-insert (path,pp) m = M.alter ins path m
-  where
-    ins Nothing    = Just $ S.singleton pp
-    ins (Just set) = Just $ S.insert pp set
-
 -- | The middleware to push files based on Referer:.
 --   Learning strategy is implemented in the first argument.
---   Learning information is kept for 30 seconds.
+--
+--   Cache of learning information is kept for 30 seconds
+--   and cleared completely.
+--   Max number of keys (e.g. index.html) is 20.
+--   Max number of values (e.g. style.css) for each key is 20.
+--   These numbers are hard-coded at this moment.
 pushOnReferer :: MakePushPromise -> Middleware
-pushOnReferer func app req sendResponse = app req $ \res -> do
-    let !path = rawPathInfo req
-    m <- reaperRead cacheReaper
-    case M.lookup path m of
-        Nothing -> case requestHeaderReferer req of
-            Nothing      -> return ()
-            Just referer -> case res of
-                ResponseFile (Status 200 "OK") _ file Nothing -> do
+pushOnReferer func app req sendResponse = app req push
+  where
+    push res@(ResponseFile (Status 200 "OK") _ file Nothing) = do
+        let !path = rawPathInfo req
+        m <- reaperRead cacheReaper
+        case M.lookup path m of
+            [] -> case requestHeaderReferer req of
+                Nothing      -> return ()
+                Just referer -> do
                     (mauth,refPath) <- parseUrl referer
                     when (isNothing mauth
                        || requestHeaderHost req == mauth) $ do
@@ -91,12 +89,12 @@
                             case mpp of
                                 Nothing -> return ()
                                 Just pp -> reaperAdd cacheReaper (refPath,pp)
-                _ -> return ()
-        Just pset -> do
-            let !ps = S.toList pset
-                !h2d = defaultHTTP2Data { http2dataPushPromise = ps}
-            setHTTP2Data req (Just h2d)
-    sendResponse res
+            ps -> do
+                let !h2d = defaultHTTP2Data { http2dataPushPromise = ps}
+                setHTTP2Data req (Just h2d)
+        sendResponse res
+    push res = sendResponse res
+
 
 -- | Learn if the file to be pushed is CSS (.css) or JavaScript (.js) file
 --   AND the Referer: ends with \"/\" or \".html\" or \".htm\".
diff --git a/Network/Wai/Middleware/Push/Referer/LimitMultiMap.hs b/Network/Wai/Middleware/Push/Referer/LimitMultiMap.hs
new file mode 100644
--- /dev/null
+++ b/Network/Wai/Middleware/Push/Referer/LimitMultiMap.hs
@@ -0,0 +1,40 @@
+{-# LANGUAGE BangPatterns #-}
+
+module Network.Wai.Middleware.Push.Referer.LimitMultiMap where
+
+import Data.Map (Map)
+import qualified Data.Map.Strict as M
+import Data.Set (Set)
+import qualified Data.Set as S
+
+data LimitMultiMap k v = LimitMultiMap {
+      limitKey :: !Int
+    , limitVal :: !Int
+    , multiMap :: !(Map k (Set v))
+    } deriving (Eq, Show)
+
+isEmpty :: LimitMultiMap k t -> Bool
+isEmpty (LimitMultiMap _ _ m) = M.null m
+
+empty :: Int -> Int -> LimitMultiMap k v
+empty lk lv = LimitMultiMap lk lv M.empty
+
+insert :: (Ord k, Ord v) => (k,v) -> LimitMultiMap k v -> LimitMultiMap k v
+insert (k,v) (LimitMultiMap lk lv m)
+  | siz <  lk = let !m' = M.alter  alt k m in LimitMultiMap lk lv m'
+  | siz == lk = let !m' = M.adjust adj k m in LimitMultiMap lk lv m'
+  | otherwise = error "insert"
+  where
+    siz = M.size m
+    alt Nothing          = Just $ S.singleton v
+    alt s@(Just set)
+      | S.size set == lv = s
+      | otherwise        = Just $ S.insert v set
+    adj set
+      | S.size set == lv = set
+      | otherwise        = S.insert v set
+
+lookup :: Ord k => k -> LimitMultiMap k v -> [v]
+lookup k (LimitMultiMap _ _ m) = case M.lookup k m of
+  Nothing  -> []
+  Just set -> S.toList set
diff --git a/wai-http2-extra.cabal b/wai-http2-extra.cabal
--- a/wai-http2-extra.cabal
+++ b/wai-http2-extra.cabal
@@ -1,5 +1,5 @@
 Name:                wai-http2-extra
-Version:             0.0.1
+Version:             0.0.2
 Synopsis:            WAI utilities for HTTP/2
 License:             MIT
 License-file:        LICENSE
@@ -22,6 +22,7 @@
                    , warp
                    , word8
   Exposed-modules:   Network.Wai.Middleware.Push.Referer
+  Other-modules:     Network.Wai.Middleware.Push.Referer.LimitMultiMap
   Ghc-Options:       -Wall
 
 Test-Suite doctest