diff --git a/ChangeLog b/ChangeLog
index 14881a0a4d..7feb15a7fc 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+Fri Aug  8 01:53:37 2014  Masaki Matsushita <glass.saga@gmail.com>
+
+	* lib/securerandom.rb: use OpenSSL::BN for performance improvement.
+
+	* benchmark/bm_securerandom.rb: benchmark script.
+
 Fri Aug  8 17:19:57 2014  SHIBATA Hiroshi  <shibata.hiroshi@gmail.com>
 
 	* lib/open-uri.rb: remove needless condition for old ruby version.
diff --git a/benchmark/bm_securerandom.rb b/benchmark/bm_securerandom.rb
new file mode 100644
index 0000000000..a082ea6d5b
--- /dev/null
+++ b/benchmark/bm_securerandom.rb
@@ -0,0 +1,5 @@
+require "securerandom"
+
+20_0000.times do
+  SecureRandom.random_number(100)
+end
diff --git a/lib/securerandom.rb b/lib/securerandom.rb
index 3ff5075023..29368ee431 100644
--- a/lib/securerandom.rb
+++ b/lib/securerandom.rb
@@ -214,21 +214,29 @@ module SecureRandom
   #
   def self.random_number(n=0)
     if 0 < n
-      hex = n.to_s(16)
-      hex = '0' + hex if (hex.length & 1) == 1
-      bin = [hex].pack("H*")
-      mask = bin[0].ord
-      mask |= mask >> 1
-      mask |= mask >> 2
-      mask |= mask >> 4
-      begin
-        rnd = SecureRandom.random_bytes(bin.length)
-        rnd[0] = (rnd[0].ord & mask).chr
-      end until rnd < bin
-      rnd.unpack("H*")[0].hex
+      if defined? OpenSSL::BN
+        OpenSSL::BN.rand_range(n).to_i
+      else
+        hex = n.to_s(16)
+        hex = '0' + hex if (hex.length & 1) == 1
+        bin = [hex].pack("H*")
+        mask = bin[0].ord
+        mask |= mask >> 1
+        mask |= mask >> 2
+        mask |= mask >> 4
+        begin
+          rnd = SecureRandom.random_bytes(bin.length)
+          rnd[0] = (rnd[0].ord & mask).chr
+        end until rnd < bin
+        rnd.unpack("H*")[0].hex
+      end
     else
       # assumption: Float::MANT_DIG <= 64
-      i64 = SecureRandom.random_bytes(8).unpack("Q")[0]
+      if defined? OpenSSL::BN
+        i64 = OpenSSL::BN.rand(64, -1).to_i
+      else
+        i64 = SecureRandom.random_bytes(8).unpack("Q")[0]
+      end
       Math.ldexp(i64 >> (64-Float::MANT_DIG), -Float::MANT_DIG)
     end
   end