diff --git a/Dockerfile b/Dockerfile
index aa3cbe2..1594480 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -7,6 +7,7 @@ RUN set -ex \
       make \
       gcc \
       libc-dev \
+      git \
  && bundle config set --local deployment "true" \
  && bundle config set --local without "development test" \
  && bundle install \
diff --git a/Gemfile b/Gemfile
index 42424c6..1c35f09 100644
--- a/Gemfile
+++ b/Gemfile
@@ -1,7 +1,9 @@
 source "https://rubygems.org"
 
+git_source(:github) { |name| "https://github.com/#{name}.git" }
+
 gem "sinatra"
-gem "sisimai"
+gem "sisimai", github: "sisimai/rb-sisimai", ref: "v4.25.16p1"
 gem "webrick"
 
 group :test do
diff --git a/Gemfile.lock b/Gemfile.lock
index 92d046d..d4f7983 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -1,21 +1,31 @@
+GIT
+  remote: https://github.com/sisimai/rb-sisimai.git
+  revision: 7d972e18862ab2c07a181e2839b4e78505d9d2c8
+  ref: v4.25.16p1
+  specs:
+    sisimai (4.25.16p1)
+      oj (>= 3.0.0)
+
 GEM
   remote: https://rubygems.org/
   specs:
     base64 (0.2.0)
+    bigdecimal (3.1.6)
     byebug (11.1.3)
     coderay (1.1.3)
-    diff-lcs (1.5.0)
+    diff-lcs (1.5.1)
     method_source (1.0.0)
     mustermann (3.0.0)
       ruby2_keywords (~> 0.0.1)
-    oj (3.15.1)
+    oj (3.16.3)
+      bigdecimal (>= 3.0)
     pry (0.14.2)
       coderay (~> 1.1)
       method_source (~> 1.0)
     pry-byebug (3.10.1)
       byebug (~> 11.0)
       pry (>= 0.13, < 0.15)
-    rack (3.0.8)
+    rack (3.0.9)
     rack-protection (4.0.0)
       base64 (>= 0.1.0)
       rack (>= 3.0.0, < 4)
@@ -43,8 +53,6 @@ GEM
       rack-protection (= 4.0.0)
       rack-session (>= 2.0.0, < 3)
       tilt (~> 2.0)
-    sisimai (4.25.16)
-      oj (>= 3.0.0)
     tilt (2.3.0)
     webrick (1.8.1)
 
@@ -56,7 +64,7 @@ DEPENDENCIES
   rack-test
   rspec
   sinatra
-  sisimai
+  sisimai!
   webrick
 
 BUNDLED WITH