A choke valve is fixed to a predetermined position even after manual release, whereby an improved start and a subsequent stable operation are obtained. A fixing force and a valve opening force are applied to a choke valve shaft. Further, a pin of a choke valve lever pushes an arm piece of a throttle valve lever to slightly open a throttle valve a choke valve is in a fully open position, and the choke valve opens half way to return the throttle valve to an idle position by a valve closing spring when a complete firing of the engine is achieved. When opening the throttle valve, a cam of the throttle valve lever pushes the pin to open the choke valve, a valve opening force overcomes a fixing force near the fully open position to set the choke valve to the fully open position, and the pin separates from the cam to open and close the throttle valve without interference with the choke valve. The fixing force overcomes the valve opening force from the fully close position to a position proximate to the fully open position, and the choke valve is fixed even after manual release.