Abstract: A communication hole is bored through a partition member which divides an intake-side priming fluid chamber from a discharge-side priming fluid chamber, whereby the two chambers are connected. The communication hole serves to prevent, on a predetermined level, a backflow of a pump fluid due to siphonage caused on the intake side when pumping operation is stopped, thereby securing a priming fluid in a pump casing. A valve assembly including a valve member is disposed corresponding to the communication hole. The valve member, which is always urged to an open position, is moved to and held in a position where it is caused to close the communication hole by the action of the pump fluid flowing in a feeding direction during pumping operation. When the pump is stopped to cause the backflow of the fluid, the valve member is moved back to the open position by the urging action.