PURPOSE: To improve the fuel consumption and the acceleration performance of engine by forming a plunger chamber between the lower end of a plunger and a booster piston while forming a pressurizing chamber between a booster piston and a pump body.

CONSTITUTION: A plunger chamber 33 communicated with a working oil inlet 31 is formed between the lower end of a plunger 42 and the upper end of the large diameter section 19 of a booster pump 18. A pressurizing chamber 24 communicated with a fuel inlet 26 and a fuel injection nozzle 9 is formed between the lower end of the small diameter section 20 of the booster piston 18 and the bottom face of an inner hole 13 of a pump body 1. Power is applied to an electromagnetic actuator 38 to lower the plunger 42 so as to pressurize the working oil in the plunger chamber 33. Consequently, the booster piston 18 is lowered to boost the fuel in the pressurizing chamber 24 so as to eject the fuel into a cylinder of engine. In such a manner, the fuel consumption and the acceleration performance of engine can be improved.