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) is determined depending on the engine temperature. The start opening degree judging unit 27 judges whether the start opening degree is closer to the fully closed side or fully opened side. The motor initializing unit 28 initializes the stepping motor 11 at the fully closed side or the fully opened side if the start opening degree is at the fully closed side or the fully opened side, respectively. After choke release, a driving signal in a specified number of steps is issued periodically.