The invention provides a pump with high driving efficiency in which the number of mechanical switching valves is decreased to reduce pressure loss and increase reliability, and which is ready for high load pressure and high-frequency driving, and which increases the discharged fluid volume for one cycle of pumping. A circular diaphragm arranged on the bottom of a casing has the outer edge fixed to the casing. The diaphragm includes a piezoelectric element to move the diaphragm on the bottom surface thereof. The space between the diaphragm and the top wall of the casing serves as a pump chamber, wherein a suction channel and a discharge channel are opened to the pump chamber, the suction channel having a check valve serving as a fluid resistive element and the discharge channel being always communicated with the pump chamber, even during the operation of the pump. In the pump, the activation of the piezoelectric element is controlled by a cycle control device so as to provide the cycle of the diaphragm in which the volume and the pressure of the discharged fluid of the pump are increased.