An oil supply groove in communication with a pump room is formed above a bearing of a housing, and an open air groove in communication with atmospheric air is formed at a position rotated around the bearing by 90° from the oil supply groove. In a shank of a rotor, a branch passage branching from an oil passage formed in its axial direction to the diametrical direction of the shank and an open air passage formed in the direction perpendicular to the branch passage are formed. The branch passage and the oil supply groove communicate with each other while the open air passage and the open air groove are arranged to also communicate with each other.