diff --git a/doc/irb/irb.rd.ja b/doc/irb/irb.rd.ja
index 416dec3906..85b6536ee4 100644
--- a/doc/irb/irb.rd.ja
+++ b/doc/irb/irb.rd.ja
@@ -83,7 +83,6 @@ irb起動時に``~/.irbrc''を読み込みます. もし存在しない場合は
 できます.
 
   IRB.conf[:IRB_NAME]="irb"
-  IRB.conf[:MATH_MODE]=false
   IRB.conf[:USE_TRACER]=false
   IRB.conf[:USE_LOADER]=false
   IRB.conf[:IGNORE_SIGINT]=true
@@ -207,9 +206,6 @@ irb拡張コマンドは, 簡単な名前と頭に`irb_'をつけた名前と両
     nil: 通常モードであれば, inspect modeとなり, mathモードの時は, non
 	 inspect modeとなる.
 
---- conf.math_mode
-    参照のみ. bcモード(分数, 行列の計算ができます)かどうか?
-
 --- conf.use_loader = true/false
     load/require時にirbのfile読み込み機能を用いるモードのスイッチ(デフォ
     ルトは用いない). このモードはIRB全体に反映される.
@@ -409,4 +405,3 @@ rubyでは, 以下のプログラムはエラーになります.
 % comment-end: "\n"
 % End:
 %
-
diff --git a/lib/irb.rb b/lib/irb.rb
index ca36acd569..3c7de05289 100644
--- a/lib/irb.rb
+++ b/lib/irb.rb
@@ -48,7 +48,6 @@ require "irb/locale"
 #
 #   Usage:  irb.rb [options] [programfile] [arguments]
 #     -f                Suppress read of ~/.irbrc
-#     -m                Bc mode (load mathn, fraction or matrix are available)
 #     -d                Set $DEBUG to true (same as `ruby -d')
 #     -r load-module    Same as `ruby -r'
 #     -I path           Specify $LOAD_PATH directory
@@ -90,7 +89,6 @@ require "irb/locale"
 # as follows in an +irb+ session:
 #
 #     IRB.conf[:IRB_NAME]="irb"
-#     IRB.conf[:MATH_MODE]=false
 #     IRB.conf[:INSPECT_MODE]=nil
 #     IRB.conf[:IRB_RC] = nil
 #     IRB.conf[:BACK_TRACE_LIMIT]=16
diff --git a/lib/irb/context.rb b/lib/irb/context.rb
index f5e1a7aa6f..9ccdf744fb 100644
--- a/lib/irb/context.rb
+++ b/lib/irb/context.rb
@@ -43,7 +43,6 @@ module IRB
       @io = nil
 
       self.inspect_mode = IRB.conf[:INSPECT_MODE]
-      self.math_mode = IRB.conf[:MATH_MODE] if IRB.conf[:MATH_MODE]
       self.use_tracer = IRB.conf[:USE_TRACER] if IRB.conf[:USE_TRACER]
       self.use_loader = IRB.conf[:USE_LOADER] if IRB.conf[:USE_LOADER]
       self.eval_history = IRB.conf[:EVAL_HISTORY] if IRB.conf[:EVAL_HISTORY]
diff --git a/lib/irb/ext/math-mode.rb b/lib/irb/ext/math-mode.rb
deleted file mode 100644
index e409dbdc6a..0000000000
--- a/lib/irb/ext/math-mode.rb
+++ /dev/null
@@ -1,48 +0,0 @@
-# frozen_string_literal: false
-#
-#   math-mode.rb -
-#   	$Release Version: 0.9.6$
-#   	$Revision$
-#   	by Keiju ISHITSUKA(keiju@ruby-lang.org)
-#
-# --
-#
-#
-#
-require "mathn"
-
-module IRB
-  class Context
-    # Returns whether bc mode is enabled.
-    #
-    # See #math_mode=
-    attr_reader :math_mode
-    # Alias for #math_mode
-    alias math? math_mode
-
-    # Sets bc mode, which loads +lib/mathn.rb+ so fractions or matrix are
-    # available.
-    #
-    # Also available as the +-m+ command line option.
-    #
-    # See IRB@Command+line+options and the unix manpage <code>bc(1)</code> for
-    # more information.
-    def math_mode=(opt)
-      if @math_mode == true && !opt
-        IRB.fail CantReturnToNormalMode
-        return
-      end
-
-      @math_mode = opt
-      if math_mode
-        main.extend Math
-        print "start math mode\n" if verbose?
-      end
-    end
-
-    def inspect?
-      @inspect_mode.nil? && !@math_mode or @inspect_mode
-    end
-  end
-end
-
diff --git a/lib/irb/extend-command.rb b/lib/irb/extend-command.rb
index 6f15e6403a..064f21ba52 100644
--- a/lib/irb/extend-command.rb
+++ b/lib/irb/extend-command.rb
@@ -214,7 +214,6 @@ module IRB # :nodoc:
     @EXTEND_COMMANDS = [
       [:eval_history=, "irb/ext/history.rb"],
       [:use_tracer=, "irb/ext/tracer.rb"],
-      [:math_mode=, "irb/ext/math-mode.rb"],
       [:use_loader=, "irb/ext/use-loader.rb"],
       [:save_history=, "irb/ext/save-history.rb"],
     ]
@@ -223,7 +222,6 @@ module IRB # :nodoc:
     #
     # Context#eval_history=::   +irb/ext/history.rb+
     # Context#use_tracer=::     +irb/ext/tracer.rb+
-    # Context#math_mode=::      +irb/ext/math-mode.rb+
     # Context#use_loader=::     +irb/ext/use-loader.rb+
     # Context#save_history=::   +irb/ext/save-history.rb+
     def self.install_extend_commands
@@ -306,4 +304,3 @@ module IRB # :nodoc:
     end
   end
 end
-
diff --git a/lib/irb/init.rb b/lib/irb/init.rb
index 1184db15ea..700466beda 100644
--- a/lib/irb/init.rb
+++ b/lib/irb/init.rb
@@ -43,7 +43,6 @@ module IRB # :nodoc:
     @CONF[:LOAD_MODULES] = []
     @CONF[:IRB_RC] = nil
 
-    @CONF[:MATH_MODE] = false
     @CONF[:USE_READLINE] = false unless defined?(ReadlineInputMethod)
     @CONF[:INSPECT_MODE] = true
     @CONF[:USE_TRACER] = false
@@ -128,8 +127,6 @@ module IRB # :nodoc:
       case opt
       when "-f"
         @CONF[:RC] = false
-      when "-m"
-        @CONF[:MATH_MODE] = true
       when "-d"
         $DEBUG = true
         $VERBOSE = true
diff --git a/lib/irb/lc/help-message b/lib/irb/lc/help-message
index 5853693eb0..d43c6a1695 100644
--- a/lib/irb/lc/help-message
+++ b/lib/irb/lc/help-message
@@ -11,7 +11,6 @@
 #
 Usage:  irb.rb [options] [programfile] [arguments]
   -f		    Suppress read of ~/.irbrc
-  -m		    Bc mode (load mathn, fraction or matrix are available)
   -d                Set $DEBUG to true (same as `ruby -d')
   -r load-module    Same as `ruby -r'
   -I path           Specify $LOAD_PATH directory
diff --git a/lib/irb/lc/ja/help-message b/lib/irb/lc/ja/help-message
index 288eb5245c..1b24d14d28 100644
--- a/lib/irb/lc/ja/help-message
+++ b/lib/irb/lc/ja/help-message
@@ -10,7 +10,6 @@
 #
 Usage:  irb.rb [options] [programfile] [arguments]
   -f		    ~/.irbrc を読み込まない.
-  -m		    bcモード(分数, 行列の計算ができる)
   -d                $DEBUG をtrueにする(ruby -d と同じ)
   -r load-module    ruby -r と同じ.
   -I path           $LOAD_PATH に path を追加する.
diff --git a/man/irb.1 b/man/irb.1
index 9083fcdbe2..e980a2ebfd 100644
--- a/man/irb.1
+++ b/man/irb.1
@@ -69,9 +69,6 @@ Suppresses read of
 .It Fl -help
 Prints a summary of the options.
 .Pp
-.It Fl m
-Bc mode (load mathn, fraction or matrix are available)
-.Pp
 .It Fl r Ar library
 Same as `ruby -r'.
 Causes irb to load the library using require.