PURPOSE: To certainly prevent engine stall by carrying-out the deceleration from the aimed value of the lower number of revolution in no-load operation to the over aimed number of revolution and then carrying-out the gradual deceleration to the aimed value of the lower number of revolution after the above-described state is stabilized in the deceleration to an idle number of revolution.

CONSTITUTION: In an idle revolution-speed controller equipped with an ISC (idle speed controller) valve 24 in a side passage 22 connected so as to detour the throttle valve (not shown in the figure) in a suction-pipe 18, said ISC valve 24 performs control according to the difference between the revolution speed information Ni supplied from an engine 30 and an aimed number of revolution by a controller 28. When the number of engine revolution is reduced to the lower number of revolution in no-load operation, the number of engine revolution is reduced to the over aimed nmber of revolution higher than the aimed value in the lower number of revolution, and when the state is stabilized, the ISC valve 24 is controlled to achieve the gradual deceleration to the aimed value of the lower number of revolution.