diff --git a/README.md b/README.md
index 8d9818e51..6b106b4f3 100644
--- a/README.md
+++ b/README.md
@@ -40,7 +40,7 @@ These methods are not implemented yet.
 + taint
 + untaint
 + to_set
-+ divide
++ divide(Set#divide with 2 arity block is not implemented.)
 
 ## License
 under the MIT License:
diff --git a/mrblib/mrb_set.rb b/mrblib/mrb_set.rb
index 0b45e9a2f..4db9729ff 100644
--- a/mrblib/mrb_set.rb
+++ b/mrblib/mrb_set.rb
@@ -296,8 +296,15 @@ class Set
     h
   end
 
-  # def divide
-  # end
+  def divide(&func)
+    return to_enum :divide unless block_given?
+
+    if func.arity == 2
+      raise NotImplementedError, "Set#divide with 2 arity block is not implemented."
+    end
+
+    Set.new(classify(&func).values)
+  end
 
   def inspect
     sprintf('#<%s: {%s}>', self.class, to_a.inspect[1..-2])
diff --git a/test/mrb_set.rb b/test/mrb_set.rb
index 0234b7692..e632a68c9 100644
--- a/test/mrb_set.rb
+++ b/test/mrb_set.rb
@@ -561,8 +561,37 @@ assert("Set#classify") do
   assert_equal(Set[2,5,8], ret[2])
 end
 
-#assert("Set#divide") do
-#end
+assert("Set#divide") do
+  # arity is 1
+  set = Set.new(1..10)
+  ret = set.divide { |i| i % 3 }
+
+  assert_equal(3, ret.size)
+  n = 0
+  ret.each { |s| n += s.size }
+  assert_equal(set.size, n)
+  assert_equal(set, ret.flatten)
+
+  assert_equal(Set, ret.class)
+  assert_true(ret.include?(Set[3,6,9]))
+  assert_true(ret.include?(Set[1,4,7,10]))
+  assert_true(ret.include?(Set[2,5,8]))
+
+
+  # arity is 2
+  set = Set[7,10,5,11,1,3,4,9,0]
+  assert_raise(NotImplementedError) {
+    ret = set.divide { |a, b| (a - b).abs == 1 }
+  }
+
+  # assert_equal(4, ret.size)
+  # n = 0
+  # ret.each { |s| n += s.size }
+  # assert_equal(set.size, n)
+  # assert_equal(set, ret.flatten)
+
+  # assert_equal(Set, ret.class)
+end
 
 # taint is not implemented yet
 #assert("taintness") do