Abstract: A fuel injection system permits the use of hydraulic control oil as a pressure transmitting medium and enables reliable valve opening and closing performance regardless of operating environment. The housing of the fuel injection system is constituted by a valve main body and a cylinder main body, the flowing channel of fuel being airtightly separated from a pressure control chamber. The cylinder main body incorporates a piezoelectric element, which changes the fluid pressure of the pressure transmitting medium in the pressure control chamber, a piston, and a cylinder having an inflow/outflow hole. A needle valve is installed, penetrating the cylinder main body so that the distal end thereof is located in a needle guide cavity of the valve main body and the other end thereof is located in the cylinder.