The open and close frequency of an intake control valve 30 for controlling an intake amount at an open and close time proportion, is variably controlled in accordance with engine rotational speed or elapsed time. As a result, during control of fuel vapor treatment, biased supply to a specific cylinder of fuel vapor drawn into the engine from an absorption device 29 is suppressed, so that an averaged air-fuel ratio can be obtained for each cylinder. Therefore, combustion variation between the cylinders can be avoided, the increased engine vibration due to the resultant increase in combustion pressure variation reduced, and the deterioration in exhaust composition and vehicle driving performance can be minimized.