PURPOSE: To improve the noise resistance and operational reliability of a relay stick detecting device by isolating a connection signal input circuit and stick discrimination circuit of the device from each other.

CONSTITUTION: The contact signal of a relay to be tested is fetched from a signal input terminal 1 and connected to the input terminal of a photocoupler 6 through protective resistances 2 and 3. The contact signal is further connected to a stick discrimination circuit 9 by means of the optical signal of the photocoupler 6 and outputted from an output terminal 10 after the propriety of the contact signal is discriminated. Sine the grounding for input is separated from the grounding for output, the above-mentioned purpose can be achieved.