PURPOSE: To provide a fluid pressure generator by which damage of a piston or the like can be prevented and additional liquid can be accurately delivered.

CONSTITUTION: The whole area of end surfaces 3a and 3b of a piston 3 is formed as a plane, and the whole area of a piston working face 3a except an opening part of a communicating passage 10 of a piston working face of a regulating body 6 is formed as a plane, and a throttle valve 14 is arranged in the middle of a supply port 12a to supply oil pressure to an oil pressure chamber 4. Therefore, when the oil pressure is supplied to the oil pressure chamber 4, the piston 3 is driven to the pressure chamber 2 side in response to this, and since additional liquid is delivered from the pressure chamber 2 at the same time, the delivery timing of the additional liquid becomes accurate, so that a proper quantity of additional liquid can be delivered according to an operating condition of a diesel engine. Driving speed of the piston 3 is restrained by the throttle valve 14, and damage of both by collision between the piston 3 and the regulating body 6 can be prevented.