PURPOSE: To prevent, in a pump whose main portion is made of resin, the destroy of a pump casing by providing in a discharge port a check valve for preventing water drop when the pump has stopped so that the internal pressure of the casing may not become higher than the closure pressure value of the pump.

CONSTITUTION: A check valve 7 is provided in a discharge port 5 to prevent water from dropping in a pump casing 3 and piping. When continuous operation is made with the stop valve 8 on discharge side opened, the temperature of the water in a pump casing rises. When pressure becomes higher than the closure pressure value of the pump, backflow is generated from discharge side to a suction port 4 so that pressure is kept at the closure pressure value. A vane 1, a guide device 2 and the casing 3 can be prevented from the deformation due to the rise of the internal pressure of the casing 3 and from mutual contact on their rotation so that operation can be stabilized.