PURPOSE: To monitor the operation of a relay contact depending on the result of calculation between an output of a monitor relay contact and a monitor control signal by providing the monitor relay contact switching the monitor signal to confirm the operation of a signal relay contact switching an input signal.

CONSTITUTION: Output signals 16-18 of monitor relay contacts go to a low level when a monitor signal 22 is at a low level and the monitor relay contacts are closed, and go to a high level when the monitor relay contacts are opened. A relay control signal 15 outputted from a control circuit 14 goes to a high level when relay contacts of relay circuits 11-13 are closed and goes to a low level when the relay contacts of relay circuits 11-13 are opened. A monitor circuit 19 ORs the output signals 16-18 of the monitor relay contacts when each relay contact of the circuits 11-13 is closed and NAND between the result of the OR arithmetic operation and a monitor control signal 23 results in a monitor result 20. When all the monitor relay contacts are closed, the level of the result 20 goes to a high level, which represents no fault in the relay operation, but when any of the monitor relay contacts malfunctions, the resulting signal 20 goes to a low level, which indicates a faulty relay operation.