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. The choke valve opening degree upon start of engine (start opening degree) is determined depending on the engine temperature. The stepping motor (11) is initialized at the fully closed side of the choke valve (9) when the power source for starting the engine is turned on. The start opening degree of the choke valve is judged to be closer to the fully closed side or fully opened side, and the stepping motor (11) is initialized at the fully closed side if judged to be close to the fully closed side, and at the fully opened side if at the fully opened side.