To provide a pump improved in the discharge pressure, a discharge rate and power efficiency and capable of controlling a flow rate in regard to a pump wherein a leakage must be prevented.

A magnet piston using permanent magnets 9 and 10 is loosely fitted in a cylinder tube 1 by using electromagnetic power, and magnetic fluxes of electromagnetic poles 2, 3 and 4 outside of the cylinder tube 1 are switched per one stroke so that the magnet piston itself reciprocate, and the magnet piston can be operated in the number of selection of the magnetic fluxes 2, 3 and 4. With this structure, digital operation and a quantitative discharge are enabled, and since the piston pump dose not have a piston rod and a ground packing, usually a piston pump has them, a fluid leakage from a pump body can be prevented and an expensive fluid and a liquid having high toxicity can be used, and since a rotary motor is unnecessary for pressure-feeding to a filter, improvement in power efficiency can be expected.