diff --git a/lib/fluent/plugin/buffer.rb b/lib/fluent/plugin/buffer.rb
index 3871b4f25b..fd2c889140 100644
--- a/lib/fluent/plugin/buffer.rb
+++ b/lib/fluent/plugin/buffer.rb
@@ -580,7 +580,7 @@ def takeback_chunk(chunk_id)
           chunk = @dequeued.delete(chunk_id)
           return false unless chunk # already purged by other thread
           @queue.unshift(chunk)
-          log.trace "chunk taken back", instance: self.object_id, chunk_id: dump_unique_id_hex(chunk_id), metadata: chunk.metadata
+          log.on_trace { log.trace "chunk taken back", instance: self.object_id, chunk_id: dump_unique_id_hex(chunk_id), metadata: chunk.metadata }
           @queued_num[chunk.metadata] += 1 # BUG if nil
           @dequeued_num[chunk.metadata] -= 1
         end
@@ -610,7 +610,7 @@ def purge_chunk(chunk_id)
             @queued_num.delete(metadata)
             @dequeued_num.delete(metadata)
           end
-          log.trace "chunk purged", instance: self.object_id, chunk_id: dump_unique_id_hex(chunk_id), metadata: metadata
+          log.on_trace { log.trace "chunk purged", instance: self.object_id, chunk_id: dump_unique_id_hex(chunk_id), metadata: metadata }
         end
 
         nil