PURPOSE: To prevent lowering of the number of revolutions of an engine and the occurrence of engine stale by providing a control means to effect control so that the duty value of a deceleration time control mechanism is adjusted to a value responding to a reduction change amount of the number of revolutions of an engine per a unit time and an intake air amount is increased.

CONSTITUTION: When a condition wherein an engine cooling water temperature exceeds a set water temperature, a condition wherein an accelerator switch 28 is turned OFF and an engine is at an decelebration period, a condition wherein the number of revolutions of an engine is below the number DPNe of starting revolutions of a deceleration time control mechanism 16, and a condition wherein a reduction change amount Ne/Time exceeds a starting differential change amount DPOT of the mechanism 16 are satisfied, the starting of the mechanism 16 is performed. During the starting of the mechanism 16, the duty value of the mechanism 16 is adjusted to a value responding to the reduction change amount Ne/Time, and control is made by a control part 18 so that an intake air amount is increased. The unnecessary starting of the mechanism 16 is blocked, operation performance is improved, and lowering of the number of revolutions of an engine and the occurrence of engine stole are prevented.