diff --git a/ChangeLog.md b/ChangeLog.md
index b3312322..b275e452 100644
--- a/ChangeLog.md
+++ b/ChangeLog.md
@@ -3,6 +3,14 @@ RELEASE NOTES for Ruby version of Sisimai
 - releases: "https://github.com/sisimai/rb-sisimai/releases"
 - download: "https://rubygems.org/gems/sisimai"
 
+v4.25.15p1
+--------------------------------------------------------------------------------
+- release: ""
+- version: ""
+- changes:
+  - Fix `Sisimai::MIME.mimedecode()` method when the encoding name is lower case
+    "b" (not "B") like `=?utf-8?b?...` #251 Thanks to @revsystem
+
 v4.25.15
 --------------------------------------------------------------------------------
 - release: "Thu, 22 Dec 2022 13:13:13 +0900 (JST)"
diff --git a/lib/sisimai/mime.rb b/lib/sisimai/mime.rb
index db189a83..72de68f1 100644
--- a/lib/sisimai/mime.rb
+++ b/lib/sisimai/mime.rb
@@ -71,7 +71,7 @@ def mimedecode(argvs = [])
             mimeencoded0   = cv[4]
 
             decodedtext0 << cv[1]
-            decodedtext0 << if encodingname == 'B'
+            decodedtext0 << if encodingname.upcase == 'B'
                               Base64.decode64(mimeencoded0)
                             else
                               mimeencoded0.unpack('M').first
diff --git a/lib/sisimai/version.rb b/lib/sisimai/version.rb
index 2d367f86..0653d34c 100644
--- a/lib/sisimai/version.rb
+++ b/lib/sisimai/version.rb
@@ -1,4 +1,4 @@
 # Define the version number of Sisimai
 module Sisimai
-  VERSION = '4.25.15'.freeze
+  VERSION = '4.25.15p1'.freeze
 end