diff --git a/test/ruby/test_settracefunc.rb b/test/ruby/test_settracefunc.rb
index 47cab56b51..3480ef2d4a 100644
--- a/test/ruby/test_settracefunc.rb
+++ b/test/ruby/test_settracefunc.rb
@@ -456,6 +456,9 @@ class TestSetTraceFunc < Test::Unit::TestCase
     bug3921 = '[ruby-dev:42350]'
     ok = false
     func = lambda{|e, f, l, i, b, k|
+      # In parallel testing, unexpected events like IO operations may be traced,
+      # so we filter out events here.
+      next unless f == __FILE__
       set_trace_func(nil)
       ok = eval("self", b)
     }