PURPOSE: To eliminate the necessity for providing a crank mechanism in a pump simplified in its constitution by forming a piston in a diagonally cut cylinder and mounting piston sliding guides, similarly formed by a diagonally cut cylinder, butt facing to a piston tilt surface in a cylinder housing this piston.

CONSTITUTION: Connecting a pair of upper and bottom cylinders C, C mutually by a flange, their upper and bottom end faces fix piston sliding guides B, B, forming one end with a predetermined tilt angle by a diagonally cut cylinder, to be arranged in a manner wherein tilt surfaces of the guides B, B are placed butt facing to each other. Mounting a piston A, formed by a diagonally cut cylinder with both upper and bottom end faces in a tilt surface, to be slidably fitted between both the sliding guides B, B, a piston rod D, connected to this piston A, is connected to a prime moving shaft G through a shaft joint E with key groove. Screw thread mounting type suction valve 4 and delivery valve 3 are mounted to the upper surface of the upper part sliding guide B. While an air vent (a) is formed in the bottom part cylinder C.