diff --git a/test/ruby/test_string.rb b/test/ruby/test_string.rb
index ed1ca895df..6581796a94 100644
--- a/test/ruby/test_string.rb
+++ b/test/ruby/test_string.rb
@@ -2258,7 +2258,7 @@ class TestString < Test::Unit::TestCase
 
   def test_LSHIFT_neary_long_max
     return unless @cls == String
-    assert_ruby_status([], <<-'end;', '[ruby-core:61886] [Bug #9709]', timeout: 15)
+    assert_ruby_status([], <<-'end;', '[ruby-core:61886] [Bug #9709]', timeout: 20)
       begin
         a = "a" * 0x4000_0000
         a << "a" * 0x1_0000