To provide the communication processor by stopping the operation of a communication processor surely to disconnect the processor from a LAN in the case that a fault takes place in one of the two communication processors connecting a host computer and the LAN and making communication with the other communication processor through the use of a same MAC address.

A diagnostic control section 16b of a communication processor 10b started in the standby system sends a diagnostic frame to a communication processor 10a started as an active system via a LAN. A diagnostic control section 16a returns a diagnostic reply frame denoting a normal state or an abnormal state in response to the diagnostic frame. When the diagnostic reply frame denoting the normal state is not received with in a prescribed time, a MAC reset section 17b receives a MAC address reset frame to delete a MAC address from an address table of a MAC control section 14a.