PURPOSE: To increase a suction flow in a restricted area by forming a suction side port by extending slenderly in the circumferential direction, in a pump in which a fluid is sucked in the piston stroke area where the suction side port opened to the side surface of a piston is communicated to a suction chamber.

CONSTITUTION: In a fixed cylinder type radial piston pump, a piston 11 in a fixed cylinder 10 is reciprocated in the diameter direction by an eccentric cam 7 which rotates and drives. The fluid is sucked in the area where a suction side port 16 opened to the side surface of the piston 11 is communicated to a suction chamber 9, within the area of stroke of the piston 11, the sucked fluid is pressurized in a cylinder chamber 15 inside the area where the suction side port 16 is being closed by the fixed cylinder 10, and the pressured fluid is exhausted from an exhaust port 18 of the cylinder chamber 15. In this case, the suction side port 16 is formed of a circular groove 16a formed at the outer side surface of the piston 11 and extending slenderly in the circumferential direction, and plural communication holes 16b which communicate the bottom of the circular groove 16a and the inner surface of the piston 11.