PURPOSE: To improve mounting and removing workability by providing a flange part for restricting the movement of a gear in the axial direction, on one end side of a cylindrical shaft part, also providing a gear cover part from the other side of the cylindrical shaft part and by fastening the cover part and the cylindrical shaft part together by a fixation bolt to a main body part.

CONSTITUTION: One end 20a of a cylindrical shaft part 20 is inserted into the recessed part 28 of a cylinder block 4, and a flange part 24 is brought into contact with the cylindrical block 4. A gear 18 is provided on the outer peripheral part of the cylindrical shaft part 20. A cover part 26 is arranged so that the other end 20b of the cylindrical shaft part 20 is fitted to the engagement recessed part 32 on the inside of a protrusion 30 of the cover part 26. The cover part 26 is fixed by a fixation bolt 22 reaching the cylinder block 4 from the mounting hole part 34 of the cover part 26 through the interior space of the cylindrical shaft part 20, and the cover part 26 and the cylindrical shaft part 20 are fastened together by the fixation bolt 22. In this way, the structure is simplified, and the mounting and removing work of the cover part 26 and the cylindrical shaft part 20 can be carried out in a short period of time by only one fixation bolt 22, and the workability is improved.