To delay the off operation of a compressor for a certain period of time and to further stably operate the compressor by outputting a high or low signal according to a voltage signal that is applied from a voltage conversion part and controlling the on/off of the compressor.

In a voltage conversion part 14, diodes D2 and D3 and a resistor R6 are connected in parallel with the output terminal of a second operational amplifier OP2, the output voltage is converted to an absolute value, is smoothed to a DC through a resistor R7 and a capacitor C2, and is applied to an input terminal A/D of a control unit 10. Although the control part 10 outputs a high signal when it operates normally, the outputted high signal is inverted to a low signal via an inverter IN and turns on a switch SW of a relay RY, and a power is supplied to a compressor 11 for driving, thus preventing the frequency on/off operation of the compressor near a critical value, and hence delaying the on/off operation of the compressor for a certain amount of time and further stably operating the compressor.