On start of an engine, a difference ΔPm between a first intake pipe pressure Pm1 and a second intake pipe pressure Pm2 is computed. Reduction correction of wall surface deposition correction is prohibited until the state that the difference ΔPm is equal to or less than a reference value ΔPmref continues over a predetermined number of strokes nref of the engine. After the state that the difference ΔPm is equal to or less than the reference value ΔPmref continues over the predetermined number of strokes nref of the engine, permission is given for the reduction correction.