PURPOSE: To prevent remarkable decrease in engine rotational speed in an internal-combustion engine for a vehicle, by allowing a bypass of a throttle valve to be communicated for a predetermined period from a timing when a compressor of an air conditioner in operation is made unoperational.

CONSTITUTION: When an engine rotational speed detected by a sensor 26 becomes higher than an upper limit of a predetermined range, a control circuit 30 acts to block a bypass 3 of a throttle valve 4 by an electromagnetic valve 5, while when the engine rotational speed becomes lower than a lower limit, the control circuit 30 acts allow communication of the bypass 3, thus controlling an idling speed within the predetermined range. Further, when operation of a compressor for an air conditioner is detected by an air conditioner switch 25, an electromagnetic valve 9 is opened to allow communication of a passage 7 thereby to increase the engine rotational speed. When the compressor is made unoperational, the passage 7 is blocked and the bypass 3 is communicated for a predetermined period. Thusly, it is possible to prevent remarkable decrease in the engine rotational speed upon unoperation of the compressor for the air conditioner.