Branch flow channels configured respectively to supply compressed air to two sieve beds respectively are provided with air supply valves each including a diaphragm valve and an electromagnetic pilot valve for driving the diaphragm valve and configured to open and close the respective branch flow channels, a pilot flow channel configured to supply pilot air to the electromagnetic pilot valves is branched from the branch flow channels, and a check valve for preventing pilot air from flowing reversely is provided in the pilot flow channel.
