To improve accelerating characteristic by discharging vapor generated inside a pump chamber to atmosphere without residing inside the pump chamber and supplying accurate accelerating fuel.

A cylinder chamber 4 is divided into a pump chamber 10 and an atmospheric chamber 11 by a plunger valve 9. A leak passage 20 through which the pump chamber 10 communicates with the atmospheric chamber 11 is formed to the plunger valve 9. The leak passage 20 is held to be open in a low opening degree region of a throttle valve by an operating lever 14. In a high opening degree of the throttle valve, the leak passage 20 is held to be closed by an operating rod 14, and the plunger valve 9 mechanically compresses the pump chamber 10 using the operating rod 14, whereby fuel inside the pump chamber 10 is boosted.