PURPOSE: To detect damage of a cable connecting transmission/reception side devices by providing a logic section discriminating the input of a significant signal when either the 1st or the 2nd voltage generated from a prescribed contact is inputted to the reception side device.

CONSTITUTION: A logical section 11 provided to the receiver side device 15 inputs a voltage V2 generated from a contact 16 of a changeover switch 8 to an input buffer 9 connected to power supply V4 through a cable 12 and an input side resistor R8. The logical section 11 inputs a voltage V3 generated from a contact 17 to an input buffer 10 connected to the power supply V4 through the input side resistor R6 and a cable 14. The logic section 11 discriminates it that a significant signal is inputted when the voltage V3 inputted from the contact 17 is a high voltage V3H and that a non-significant signal is inputted when the voltage V3 inputted from the contact 17 is a low voltage V3L, and that cables 12, 14 are damaged when both the voltages are inputted at the same time via the cables 12, 14.