To arbitrarily turn on and off the independent power sources of an input side and an output side without generating any malfunction by serially connecting a diode with the input side and output side of a differential compara tor, and preventing flow-in or flow-out currents from one power source to the other power source side.

A control input signal is impressed through a signal line from an interface 66 of a sensor 60 through a signal line to the non-inversion input terminal of an open collector type comparator 31, and a reference potential is impressed from a reference potential source 34 to an inversion input terminal. Then, a hysteresis circuit 30 generates an output when an input signal from the sensor 60 is beyond the prescribed level of the reference potential, and the output is turned into 0 when it is below the prescribed level. In this case, a diode 35 is serially connected in a forward direction from the interface 66 of the sensor 60 to the non-inversion input terminal of the comparator 31, and a diode 37 is serially connected in a forward direction from a device side power source 50 to a resistance 36. Thus, the flow-in from the device side power source 50 to the sensor 60 side and the flow-in from the sensor 60 side to a load 40 side can be prevented.