diff --git a/README.md b/README.md
index b9dcf7b..7607cde 100644
--- a/README.md
+++ b/README.md
@@ -120,6 +120,13 @@ Maximum number of worker threads.
 
 Default: number of logical cores
 
+#### `--no-use_threads`
+
+Do not use threads, process each message using raw pub/sub event
+callback.
+
+Default: use threads
+
 #### `--project_id=PROJECT_ID`, `--credentials=KEYFILE_PATH`
 
 Credentials of Google Cloud Platform. Please see [the document](https://github.com/GoogleCloudPlatform/google-cloud-ruby/blob/master/AUTHENTICATION.md) for details.
diff --git a/exe/activejob-google_cloud_pubsub-worker b/exe/activejob-google_cloud_pubsub-worker
index 65b9a2e..88cfff0 100755
--- a/exe/activejob-google_cloud_pubsub-worker
+++ b/exe/activejob-google_cloud_pubsub-worker
@@ -24,6 +24,10 @@ worker_args = parser.define_by_keywords(
   }
 )
 
+parser.on '--[no-]threads' do |o|
+  worker_args[:use_threads] = o
+end
+
 pubsub_args = parser.define_by_keywords(
   {},
   Google::Cloud::Pubsub.method(:new),
diff --git a/lib/activejob-google_cloud_pubsub/worker.rb b/lib/activejob-google_cloud_pubsub/worker.rb
index 56d4f9e..2c2269e 100644
--- a/lib/activejob-google_cloud_pubsub/worker.rb
+++ b/lib/activejob-google_cloud_pubsub/worker.rb
@@ -13,40 +13,53 @@ class Worker
 
       using PubsubExtension
 
-      def initialize(queue: 'default', min_threads: 0, max_threads: Concurrent.processor_count, pubsub: Google::Cloud::Pubsub.new, logger: Logger.new($stdout))
+      def initialize(queue: 'default', min_threads: 0, max_threads: Concurrent.processor_count, use_threads: true, pubsub: Google::Cloud::Pubsub.new, logger: Logger.new($stdout))
         @queue_name  = queue
         @min_threads = min_threads
         @max_threads = max_threads
+        @use_threads = use_threads
         @pubsub      = pubsub
         @logger      = logger
       end
 
       def run
-        pool = Concurrent::ThreadPoolExecutor.new(min_threads: @min_threads, max_threads: @max_threads, max_queue: -1)
+        if @use_threads
+          pool = Concurrent::ThreadPoolExecutor.new(min_threads: @min_threads, max_threads: @max_threads, max_queue: -1)
+        else
+          @logger&.warn "Running without threads"
+        end
 
         @pubsub.subscription_for(@queue_name).listen {|message|
           @logger&.info "Message(#{message.message_id}) was received."
 
-          begin
-            Concurrent::Promise.execute(args: message, executor: pool) {|msg|
-              process_or_delay msg
-            }.rescue {|e|
-              @logger&.error e
-            }
-          rescue Concurrent::RejectedExecutionError
-            Concurrent::Promise.execute(args: message) {|msg|
-              msg.delay! 10.seconds.to_i
-
-              @logger&.info "Message(#{msg.message_id}) was rescheduled after 10 seconds because the thread pool is full."
-            }.rescue {|e|
-              @logger&.error e
-            }
+          if @use_threads
+            run_concurrent message, pool
+          else
+            process message
           end
         }.start
 
         sleep
       end
 
+      def run_concurrent(message, pool)
+        begin
+          Concurrent::Promise.execute(args: message, executor: pool) {|msg|
+            process_or_delay msg
+          }.rescue {|e|
+            @logger&.error e
+          }
+        rescue Concurrent::RejectedExecutionError
+          Concurrent::Promise.execute(args: message) {|msg|
+            msg.delay! 10.seconds.to_i
+
+            @logger&.info "Message(#{msg.message_id}) was rescheduled after 10 seconds because the thread pool is full."
+          }.rescue {|e|
+            @logger&.error e
+          }
+        end
+      end
+
       def ensure_subscription
         @pubsub.subscription_for @queue_name