PURPOSE: To improve flexibility of the titled device, by a method wherein a converting voltage output of a number of a revolutions of an engine is applied to a comparator through a different time constant circuit and is made to revert so that a solenoid clutch is interrupted, in a separator of a compressor for an engine-driven air conditioner.

CONSTITUTION: While an internal combustion engine is in a fixed revolution or dropping mode, a negative input electric potential of a comparator 22 becomes higher than a positive input electric potential by a lowering quantity of voltage in an orderly direction of a shift dioder 25, a signal So of an output terminal 32 becomes a low level, an excitation current is electrified to a relay 33, and an electric clutch 35 is engaged. Although an output of an integrating circuit 16 and both positive and negative inputs of the comparator 22 are increased when a revolution of the engine begins to increase, the positive input electric potential becomes superior to that of the negative input and the output terminal 32 of the comparator 22 is inverted to a H level. Electrification to the relay 33, therefore, is interrupted and the clutch 35 is disengaged. Thus, constitution becomes simple, and an improvement in flexibility of the titled device can be contrived.