To surely discharge air inside a pump so as to prevent deterioration in pump performance due to stayed air in the inside of the pump.

A return passage 50 is opened and arranged in a discharge side ring type groove 27 inside a pump housing 6. A check valve 51, which is opened when an internal pressure of the discharge side ring type groove 27 is lower than the set pressure, is arranged in the return passage 50. When an operation of a pump is started, the internal pressure of the discharge side ring type groove 27 is reduced below the set pressure, so that the check valve 51 opens the return passage 50, and air inside the pump is automatically discharged.