To provide an injector to enable the adjustment of fuel injection timing and the metering of fuel injection quantity with high precision.

In the injector 1, on the fuel injection, a small diameter piston 14 is moved to a control valve 9 side, and the control valve 9 is pushed by a rod 14a with the movement to open the valve. High pressure fuel in a pressure control chamber 5 is discharged from a discharge passage 8 with the opening of the control valve 9 so that a needle valve 3 is moved to the position to open an injection hole 2, and fuel in a fuel reservoir 6 is injected via the injection hole 2. A through-hole 16a penetrated by a rod 14a is bored between the discharge passage 8 and a piston housing 13, and a shielding plate 16 to separate the piston housing 13 from the discharge passage 8 is installed. The outside diameter of the rod 4a and the inside diameter of the through-hole 16a are set to such a value that both are slidably contact with each other so that high pressure fuel in the discharge passage 8 can be prevented from permeating into the piston housing 13 via between the outside perimeter of the rod 14a and the inside perimeter of the through-hole 16a.