PURPOSE: To discriminate the presence of the connection of an external device automatically and quickly by applying a low voltage divided by resistors to an interface bus at a master device and applying a voltage different from that given to the master device to an interface bus at an external device side.

CONSTITUTION: When the external device 2 is not connected to the master set 1 via the interface bus 3, a voltage v1 through division by resistors 4a, 4b is applied to the interface bus 3 and it is selected to be higher than a reference voltage v0. A voltage divided by resistors 8a, 8b of the external device 2 and applied to the interface bus 3 is selected lower than the voltage v1. Thus, a control section 6 discriminates the presence of the connection of the external device 2 depending on the voltage comparison output signal of a voltage comparator 5.