To drain water, remaining in a pumping-up pipe, as rapidly as possible after cutoff of pumping up.

A casing 35 is provided with an impeller chamber 3; and a bypass flow passage 36 communicated with a flow passage 51 situated upper stream from an impeller. A valve mechanism 112 to open and close a bypass flow passage 35 comprises a hollow ring body 110 having a water passage port 112; and a resilient tube body 120. A fluid feed and discharge mechanism 200 is provided to feed and discharge fluid to and from an elastic tube body 120.