PURPOSE: To obviate the operation of a switch during connection of a device and to sharply reduce a working time during mounting by a method wherein connection of an auxiliary device is detected and display responding to the connected auxiliary device is effected.

CONSTITUTION: In mutual communication through a communication line 7 between a main device 3 and an auxiliary device 5, the auxiliary device 5 to receive a connection and disconnection confirming command automatically issued from the main device 3 through turning ON of a power source is formed so as to reply a connection command in a given time. Connection to and disconnection from auxiliary devices 5A-5C and information on the connected auxiliary device 5 can be easily confirmed through display of a display device 1. Since confirmation of system constitution and a content is effected each time a power source is turned ON, failure in operation of the auxiliary device 5 or defective connection of the communication line 7 can be confirmed at each of the connected auxiliary devices 5A-5C.