To conduct maintenance of a private branch exchange through remote control without the need for provision of a leased channel or an exclusive terminal for the maintenance.

A multi-functional telephone set 4 dials an incoming telephone number of a private branch exchange B2 and dials its sub address succeedingly. A private branch exchange A2 generates a message including a caller number of the multi-functional telephone set 4 together with the incoming telephone number and the sub address and transmits the message to the private branch exchange B2. The private branch exchange B2 compares the sub address included in the received message with an access code that has been set in advance in a database and conducts processing in a system maintenance mode or in a system monitor mode according to the result of comparison in cooperation with the private branch exchange A2 by transmitting a message to the private branch exchange A2.