A pilot injection system according to the present invention, which is used in a fuel injection pump, has a casing attached to the pump. The casing connects internally with a pump chamber of the injection pump. A partition wall is fixed in the casing, whereby the pump chamber is divided from the inside of the casing. A movable wall, which is movably located in the casing, divides the inside space of the casing between a fuel-spill chamber and a holding chamber. The partition wall is formed with a valve hole, which connects the pump chamber and the spill chamber. The valve hole has a seat surface which faces the pump chamber. A valve plug, which is located in the pump chamber, can be seated on the seat surface. The valve plug is connected to the movable wall by means of a coupling rod, which extends through the valve hole. Located in the spill chamber is a return spring which urges the movable wall in a direction such that the valve plug closes the valve hole. Moreover, an actuator for moving the movable wall is disposed in the holding chamber of the casing. The actuator includes a stack of piezoelectric elements, which can extend or contract in accordance with a voltage applied thereto.