The fuel injection system for a diesel engine comprises a pintle type fuel injection nozzle having a valve needle slidable back and forth in the axial direction thereof between a first position in which its front end portion is inserted in a spray hole and a second position in which its front end portion is retracted from the spray hole, fuel supply means for feeding pressurized fuel to the fuel injection nozzle, and a plunger member which is slidable in the axial direction of the valve needle and provided with a front end face opposed to the rear end face of the valve needle and a rear end face adapted to receive the pressure of the pressurized fuel fed to the fuel injection nozzle from the fuel supply means, the valve needle being adapted to be lifted by a predetermined fuel pressure to a pre-lift position where the rear end face of the valve needle abuts against the front end face of the plunger member so that further lift of the valve needle is suppressed by the fuel pressure acting on the rear end face of the plunger member, thereby changing the valve opening pressure stepwise with change in the amount of lift of the valve. The injection system is provided with pre-lift change means for changing the amount of the pre-lift of the valve needle, an operating condition detecting means for detecting the operating condition of the engine, and a control means for controlling the pre-lift change means according to the output of the operating condition detecting means.