A bellows that is extendingly and contractingly deformable in the axial direction is placed in a pump body, and a liquid chamber is formed inside the bellows. A suction port and a discharge port are formed in an inner bottom face of the pump body facing the liquid chamber. Liquid is sucked from the suction port into the liquid chamber by extension of the bellows, and the liquid in the liquid chamber is discharged from the discharge port by contraction of the bellows. In each of a suction ball type check valve disposed in the suction port, and a discharge ball type check valve disposed in an outflow passage which communicates with the discharge port, a cylindrical valve casing is disposed, and ball valve elements in the valve casing are caused by their own weight to be closely contacted with valve seat, thereby preventing liquid from reversely flowing.