To provide a variable displacement piston pump which has increased pump efficiency by more approximating pump output: H=P×q to an ideal fixed output line.

A pressure reducing valve 37 attached to a body 11 includes: a valve body 45 which is integrally attached to the body 11 by a bolt member (that is not illustrated); a sleeve 46 which is screwed to the valve body 45 while being opposed to a cylinder 32 coaxially with the cylinder 32; a spool 47 which is freely slidable fitted into the sleeve 46; a spring holder 48 which is provided adjacently to the sleeve 46 and mounted to a housing 11 and the valve body 45; and a spring member 50 which is inserted between the spring holder 48 and a spring guide 49 screwed to an axial end (right end in Fig.2) of a rod 35.