A communication system 10 is comprised of a communication device 11 and a relay device 12 that is connected to the communication device 11 by radio at close range and an other communication device 15 via a network and that relays a communication between the first device 11 and the other communication device 15. Such communication system 10 comprises local communicating units 112 and 121 and a network communicating unit 123 that detect a disconnection of a radio channel between the communication device 11 and the relay device 12 and a disconnection of a radio channel between the relay device 12 and the other communication device 15. The communication device 11 includes a notifying unit 113 that notifies a user of that a disconnection of which radio channel of a plurality of the radio channels is detected.