PURPOSE: To sharply decrease power transferred from a pump to an engine so as to improve decelerating properites of the engine, by a method wherein flow-in of feed air, being about to enter an operating chamber until the middle stage of a stroke, is completely interrupted or sufficiently decrease in its amount during deceleration of an engine.

CONSTITUTION: During deceleration of an engine, a throttle valve 13 is brought to its minimum opening position, and both a throttle switch 24 and a number of revolution detecting switch 23 are turned ON. A negative pressure circuit 21 is opened by an electromagnetic valve 22, a intense negative pressure is introduced to a diaphragm device 19, a reduction valve 18 is fully closed or nearly fully closed, and the flow-in of feed air, which is about to enter an operating chamber 7 until the middle stage of a stroke, is perfectly interrupted or enoughly decreased in its amount. This causes a pressure at the middle stage of a stroke in the operating chamber 7, to be approximately equalized with a pressure (a negative pressure) on the discharge side of a pump 2, resulting in the generation of almost no power at the pump 2.