PURPOSE: To prevent abnormal noise even under transient phenomena by placing a separating valve for uniforming the sealing liquid to be supplied into pump chamber at the midway of sealing liquid supply path connecting between the pump chamber and liquid source.

CONSTITUTION: Separating valve 25 is provided at the midway of sealing liquid supply path connecting between pump chambers 4, 5 and liquid source 27. Consequently when the pressurized sealing liquid from liquid source 27 is supplied through the gaps 9, 10 between the impellers 4, 5 and port plates 2, 3 into pump chambers 4, 5, same amount of sealing liquid is supplied into liquid compensation chambers 22, 23 thereby causes no unballance. Consequently water hummer never occurs even when transient phenomena is applied on the pump resulting in continuous stable operation.