PURPOSE: To facilitate flow control by connecting a volume control chamber to a pump chamber and adjusting the retreat restricting position of a volume control piston in the volume control chamber, utilizing the pressure in a discharge pipe.

CONSTITUTION: A volume control chamber 4 is adjacently provided to a piston cylinder 2, a volume control piston 9 is fit in the volume control chamber 4 and a support member 9' for restricting the retreat position of the piston 9 is provided behind it. The rise in pressure in a discharge pipe 20 operates a flow control piston 13 through a branch pipe 19 and a cam profile 12 allows an adjusting control rod 10 to project to back the retreat restriction position of the volume control piston 9, while absorbing the change in the volume of a pump cylinder 2 to reduce the rate of flow to the discharge pipe 20 by a pump piston 6. Thus, flow control is facilitated.