To stabilize operation performance of an engine by providing a holding means to constrain the valve element of a shut-off valve in an open valve position, and making a structure of this holding means so as to release constraint of the valve element by a valve lever when a throttle valve is opened to a certain opening degree.

If a linear movement part 31 is pulled with a knob against a spring 33 in advance of starting an engine, a valve element 28 is separated from a valve seat 27, and an air passage, a fuel passage part 23 and a mixed air passage part 24 composing an auxiliary passage 21 are connected in series. At this time, a constraining part 38 is moved in a direction to be released from a cut-out groove 37, and is fitted in an engagement groove 36 by a constraining spring 39 when the valve element 28 reaches a fully opened position, so that the linear movement part 31 is constrained in an open valve position. If cranking is carried out under this condition, air and fuel are suctioned by a negative pressure of intake air through the auxiliary passage 21, and is introduced into an intake air passage 5. After that, when a throttle valve 4 is opened wider than a partially opened position, constraint of the linear movement part 31 is released, and the valve element 28 is closed.