PURPOSE: To discover disconnection and short-circuiting by checking for terminal potentials at one end of a plurality of wirings under various conditions, where the application of a voltage or grounding is made possible through a resistance while the other end thereof is allowed to be grounded with an earth conductor.

CONSTITUTION: Wirings 2aW2c are free one end thereof and will be short- circuited between each other with an earth conductor 7 in a disconnection test. At the other end, the wiring are connected to a receiver 3 through connectors 8 and 9. Input wires AWC of the receiver 3 are connected to a test voltage source separately through resistances 11aW11c and 10aW10c while allowed to be grounded through the resistances 11aW11c and switches 6aW6c. This enables the setting of levels of the terminals AWC depending on the state of the switches 6aW6c. Based on the setting, a test for disconnection and short-circuiting can be done.