To improve sound vibration performance and volumetric efficiency by making the delivery stroke of a radial piston pump follow an accurate sign curve.

An eccentric cam 9 formed on an input shaft is arranged in an intake chamber 5 communicating with a suction port of a pump housing 1, cylinder bores 12a to 12d are projectingly and radially provided around the eccentric cam 9 at equal distances, suction pistons 14A, 14B provided with suction holes 13 are arranged on the cylinder bores 12a, 12c, and compression pistons 15A, 15B having no suction hole are arranged on the cylindrical bores 12b, 12d, so as to be freely moved by the eccentric cam 9. Pressure chambers 18A, 18B, 19A, 19B sectioned by the pistons 14A, 14B, 15A, 15B communicate by communication passages 20A, 20B. The delivery wave from the whole pump is made into a complete sine curve by allowing operating fluid to flow to the compression piston sides 15A, 15B in starting of the compression stroke of the suction pistons 14A, 14B in the communication passages.