To reduce a pump loss caused by this reduction, by reducing frictional resistance between a cam-follower and a cam, in a radial piston pump.

A plurality of pistons 15 for forming a pump room Ro are reciprocatably installed in the cylinder shaft direction in respective cylinder holes 11a arranged in a rotary shaft 11. A cam mechanism 20 is arranged between the respective pistons 15 and a cylindrical member 13 for driving the respective pistons 15 in the cylinder shaft direction in the same phase by relative rotation of the rotary shaft 11 and the cylindrical member 13. A cam chamber R1 for arranging the cam mechanism 20 and storing an end part of the respective pistons 15 is sealed, and a required quantity of lubricating oil is stored in this cam chamber R1. An air chamber R2 is arranged for reducing a volumetric variation in the cam chamber R1.