To secure excellent engine restartability even in the state where fuel leaks out into an intake passage from an acceleration pump after engine stop.

The overrich prevention mechanism includes an outside air introducing passage 11 for communicating an air side to an intake passage, a temperature sensing type valve opening/closing means for closing a first cutoff valve 12 arranged in the outside air introducing passage 11 through the deformation thereof by a valve element supporting member 12b at a designated temperature or higher, and a complete explosion diaphragm 13 taken as a boost pressure sensing type valve opening/closing means for closing a second cutoff valve 130 arranged in the outside air introducing passage 11 when introduced engine boost pressure exceeds a prescribed level. At engine restart in a state where fuel in an acceleration pump mechanism leaks out into an intake passage after engine stop, the overrich state is avoided through the introduction of outside air into the intake passage by leaving the outside air introducing passage 11 open until the engine boost pressure exceeds a prescribed level.