A choke valve is controlled finely suited to the running state of an engine. A throttle valve 8 and a choke valve 9 are provided in series on an intake pipe 6, and the opening degree of the choke valve 9 is controlled by a stepping motor 11. A choke valve opening degree upon start of engine (start opening degree) and a driving pulse rate of a stepping motor 11 are determined depending on the engine temperature. When lower than the engine temperature TL, the pulse rate is set at first rate, or the lowest rate in a specified range. Depending on the engine temperature, the pulse rate is set higher gradually up to second rate TH. When releasing the choke gradually in warm-up operation, the pulse rate is lowered so as to obtain a choke opening degree of high precision by a high torque.