To prevent the size of an oil pump from being increased by securing the height of an oil passage perpendicular to the axis of a shaft of an annular passage provided in a mutually fitting part of a main shaft and a stator shaft without reducing the strength of the main shaft.

An annular passage R to be provided at a mutually fitting part of a stator shaft 4 and a main shaft 5 is formed of a groove of the shaft 5 passed in a hollow hole of the shaft 4 and a groove of the shaft 5 press fitted to a pump cover 1b in an opposite manner to each other, and each side of the annular passage R is sealed by a seal ring 7 to secure the sufficient height of the oil passage without reducing the strength of the shaft 5 and without increasing the size of an oil pump 1.