To enhance ignitability for an air-fuel mixture to prevent generation of detective starting by bringing a gas fuel ratio in the air-fuel mixture into higher level than that in stationary operation at the starting time of an engine of a lean burn type.

An air volume regulating valve 7 is provided inside a duct 4 constituting an air inflow passage for a gas mixer 2. An intake temperature data and an oil temperature data detected by an intake temperature sensor 18 and an oil temperature sensor 19 are input to a control unit 10. The control unit 10 rotates a motor 17 via a motor driving circuit 14 when the unit 10 determines that the intake temperature data and the oil temperature data are lower than a reference temperature to deteriorate startability at the starting time of an engine 8, operates the air volume control valve 7 to reduce the opening area of the duct 4, and raises a gas fuel ratio in an air-fuel mixture by reducing air in-flow volume to the mixer 2 to enhance startability.