PURPOSE: To allow the user to purchase a slave set without limitation by providing a function to write a same ID code as a master set into the slave set to the master set.

CONSTITUTION: Operation sections 1d, 2d are operated for ID code write, control circuits 1a, 2a of a master set 1 and a slave set 2 read a number (n) registered in the slave set 2 precedingly from a memory and sets a number (n+1) to be registered to the slave set 2 this time. Then the number (n+1) is displayed on a display device 1e. When a switch is operated by the operation section 1d to send an ID code of the master set 1, an ID code corresponding to the number (n+1) is loaded from an ID ROM 1b and the loaded ID code is sent through a connection cable 3. Upon the receipt of the ID code from the master set 1, the control circuit 2a of the slave set 2 stores the received ID code to an ID RAM 2b and sent to the master set 1 through the connection code 3.