PURPOSE: To prevent dispersion of an injection quantity of high pressure fuel and suppress oscillation of a diesel engine by preventing generation of unevenness at the time of main injection within a pilot required injection interval under the normal operational condition of the diesel engine.

CONSTITUTION: A second valve 8 seated on a stepped shoulder part 56 is slidably provided inside a main back pressure chamber 52, and a second orifice 83 for communicating the main back pressure chamber 52 with a communication passage 66 is formed in the second valve 8. A flow passage on the outer circumference of the second valve 8 is eliminated by providing a cylindrical seal part 84 in slidably contact with the inner circumferential face of a nozzle holder 5, on the outer circumferential face of the second valve 8. Additionally, the spring force of a coil spring 9 for energizing the second valve 8 to the side of stepped shoulder part 56 is remarkably weakened, and the second valve 8 is not seated on the stepped shoulder part 56 within a pilot required injection interval under the normal operation condition of a diesel engine, and then a convex type injection rate is achieved at the time of main injection.