To prevent fluid from being uselessly discharged, while accurately restraining reduction in a delivery quantity in the initial stage of a delivery period, which is generated by looseness of a mechanical system, in a nonpulsating pump.

The reduction in the delivery quantity in the initial stage of the delivery period of a diaphragm pump 12, is prevented by reducing the volume of a pump room 18, by driving a plunger 16 in the delivery direction just before its period. The reduction in this pump room volume is set larger than a reduction quantity of a supposed delivery quantity, and a difference between an actual reduction quantity and the largely set reduction quantity, is offset by a volume change in a volume adjusting mechanism 40 of changing the volume by pressure in the pump room.

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