A pump is provided having a pump chamber that is capable of changing a volume thereof by a diaphragm, an inlet passage permitting an operating fluid to flow into the pump chamber; an outlet passage permitting an operating fluid to flow out from the pump chamber; and a check valve provided on the inlet passage. An inertance value of the inlet passage is smaller than the inertance value of the outlet passage. The diaphragm is driven by a frequency f (Hz) satisfying the following formula:.gtoreq..times..times..times. ##EQU00001## wherein the inertance value of the outlet passage is L (kg/m.sup.4), the displacement from an upper end point to a bottom end point of the diaphragm is X (m), and the cross section area of the pump chamber is S (m.sup.2).