diff --git a/lib/fluent/supervisor.rb b/lib/fluent/supervisor.rb
index 750bd1efb7..ac2c249b28 100644
--- a/lib/fluent/supervisor.rb
+++ b/lib/fluent/supervisor.rb
@@ -173,7 +173,9 @@ def supervisor_sigusr1_handler
 
     def kill_worker
       if config[:worker_pid]
-        config[:worker_pid].each do |pid|
+        pids = config[:worker_pid].clone
+        config[:worker_pid].clear
+        pids.each do |pid|
           if Fluent.windows?
             Process.kill :KILL, pid
           else