To obtain a piston of a compact structure without using a U-shaped spring nor a piston return spring.

End faces of two pistons 22 are abutted against an outer peripheral surface of a rotationally driven eccentric body. The pistons are axially moved in a manner of reciprocating strokes by rotationally driving the eccentric body. A push-away chamber are formed in each of the pistons, which is alternatively widened and contracted by reciprocatingly stroking the piston. Such a piston pump 10 has a return chamber 60 for respective piston, whose capacity is alternatively widened and contracted by reciprocatingly stroking the piston 22. When the return chamber 60 of one piston 22 is contracted, the chamber 60 of the other piston 22 is widened, and vise versa. The return chambers 60 mutually communicate with each other.