diff --git a/chainermn/optimizers.py b/chainermn/optimizers.py
index 901e7c9e..55eaf24e 100644
--- a/chainermn/optimizers.py
+++ b/chainermn/optimizers.py
@@ -1,6 +1,18 @@
 import chainer
 import copy
 import multiprocessing.pool
+import warnings
+
+
+def _check_mp_start_method(comm):
+    """Show a warning if multiprocessing's start_method is not 'forkserver'."""
+    method = multiprocessing.get_start_method()
+
+    if comm.size > 1 and comm.rank == 0:
+        if method is not 'forkserver':
+            warnings.warn("multiprocessing's `start_method` must be "
+                          "'forkserver' (now it's '{}')".format(method),
+                          stacklevel=2)
 
 
 class _MultiNodeOptimizer(object):
@@ -164,6 +176,8 @@ def create_multi_node_optimizer(actual_optimizer, communicator,
     Returns:
         The multi node optimizer based on ``actual_optimizer``.
     """
+    _check_mp_start_method(communicator)
+
     if double_buffering:
         from chainermn.communicators.pure_nccl_communicator \
             import PureNcclCommunicator