PURPOSE: To ensure a constant delivery amount to be maintained at all times by providing a magnetic substance piston having the capability of giving displacement over constant stroke in state energized toward a piston receiver, and having a through- hole as well as a valve retainer at the side of the piston receiver.

CONSTITUTION: A magnetic flux generated at a magnetic force supply source 20 passes a piston 13 of magnetic substance after a magnetic substance cylinder section 4b, thereby causing the piston 13 to be displaced over constant stroke toward a suction cylinder section 5. As a result, a ball valve 15 parts from a valve retainer 12 against the force of a spring 14, and a fluid in a piston receiver 9 is drawn from the suction cylinder section 5 via the through-hole 11 of the piston 13. Also, the ball valve 15 closes the valve retainer 12, due to the energizing force of the spring 14. Then, when the supply of the magnetic flux from the source 20 is interrupted, the piston 13 is reset to an initial position, due to the energizing force of a spring 10, and a ball valve 17 parts from a valve retainer 8, due to the internal pressure of the piston receiver 9. Thus, a fluid is delivered through a delivery cylinder section 6 by every constant amount determined by capacity corresponding to piston stroke. When the delivery of a fluid is completed, the valve 17 closes the valve retainer 8 and initial state is restored.