PURPOSE: To save wires for the device and to attain long service life by changing over the operating state of a camera section into the pre-heat state and the normal operating state by means of one cable for supplying video signal from the camera section for supervision to a monitor section.

CONSTITUTION: A commercial power supply is lowered a voltage by a transformer 4 in the monitor section 1 at first, the power is rectified and smoothed by a rectifier and smoothing circuit 5 to obtain a DC power supply. On the other hand, a reference voltage is formed by a reference voltage generating circuit 12 from a voltage applied via a cable 3 in the camera section 2. In order to changeover the state, a voltage discriminating circuit 17 compares a reference voltage from the reference voltage generating circuit 12 with a voltage dividing a supply voltage from the cable 3 at first to discriminate whether the voltage applied from the monitor section 1 via the cable 3 is a stand-by voltage or the operating voltage. Further, when the stand-by voltage is applied, the circuit is changed over so that the stand-by changeover circuit 18 is controlled and a pre-heat voltage is generated from a per-heat voltage generating circuit 13.