To improve assemblage performance in fitting and engaging a cylinder member into an inner hole in a pump housing to secure seal performance between the cylinder member and the inner hole in the pump housing, and restrict deformation of a cylinder hole.

A piston pump is provided with the pump housing 11 having an intake port 11a and a delivery port 11b, the cylinder member 13 to be inserted into the inner hole 11c in the pump housing to be assembled, a piston 15 to be reciprocated in the axial direction to form a pump chamber Ro in a cylinder hole 13a in the cylinder member, an intake valve V1 opened when the volume of the pump chamber Ro is increased, and closed when the volume is decreased, and a delivery valve V2 opened when the volume of the pump chamber Ro is decreased, and closed when the volume is increased. An insertion side end part of the cylinder member 13 has smaller diameter than the inner hole 11c in the pump housing 11. At the outer circumference of the cylinder member 13, except for the insertion side end part of the smaller diameter, a circular projection 13e having a smooth outline form is provided. The circular projection 13e is fitted and engaged into the inner hole 11c in the pump housing 11.