PURPOSE: To provide a centrifugal pump built-in with a gas vent valve formed in small size by simplifying a structure further with pump operability excellent.

CONSTITUTION: Relating to a centrifugal pump, an impeller 20 of a pump part 18 is mounted on a rotary shaft 16 in a motor part 14 journalled by bearings 10, 12, to lubricate and cool the bearing and the motor part through a flushing liquid supplied from a flushing liquid supply port 26, thereafter is release this flushing liquid to a suction side of the pump part. Accordingly, by providing a discharge path 52 with a check valve 50 between a flushing liquid reservoir (front rotor chamber 42) of the motor part 14 and a delivery side of the pump part 18, the constitution is provided so as to discharge gas in the motor part into the pump part through a flushing liquid pressure. Further, the flushing liquid pressure is set higher than a pump suction pressure and lower than a pump delivery pressure.