PURPOSE: To enable relief of the starting load of a compressor, by a method wherein a differential pressure between the high and the lower pressure side is detected, and when the differential pressure is high, after the differential pressure is decreased by opening a bypass circuit, a compressor is driven.

CONSTITUTION: A differential pressure switch 7, which is actuated by means of a differential pressure between the high and the low pressure side of a compressor, is connected to an A-contact 7A side when the differential pressure is high. When, with this state, a thermostat 5 is brought to an ON state, an electromagnetic valve 6 is energized through a normally-closed contact 4A, and the electromagnetic valve 6 is released to intercommunicate the high and the lower pressure side by means of a bypass circuit. When pressure on the high and the lower pressure side are balanced with each other, a B-contact 7B of the differential pressure switch 7 is closed, when an electromagentic coil 3 is energized, an operation switch 2, simultaneously, is closed to start operation of a compressor. Since the normally-closed contact 4A is self-held, an electromagnetic valve 6 in an electric bypass circuit is prohibited against opening during normal operation.