To accurately restrain reduction in a delivery quantity in the initial stage of a delivery period caused by looseness of a mechanical system, in a non-pulsation pump using a reciprocating pump.

The reduction in the delivery quantity in the initial stage of the deliver 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 the volume of the pump room 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. A quantity (an adjusting quantity) of the volume change in the volume adjusting mechanism 40 is set based on an average delivery pressure detected by a pressure sensor 44. The average delivery pressure and the adjusting quantity for minimizing pulsation in its pressure, are stored in a storage part 86 in advance. A control part 46 sets the adjusting quantity based on this information and the detected average delivery pressure.

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