PURPOSE: To prevent the entrance of dust and muddy water by forming a spiral groove for air vent inside a cylindrical portion in which a baring is press fitted.

CONSTITUTION: In the cylindrical portion 1a of a pump body, a hole which is set in slightly smaller diameter than the outside diameter of the outer race of a bearing 2 which is press fitted therein is formed, and an internal thread type spiral groove 1d is directly formed in this hole. After that, an input shaft 3 and the bearing 2 fitted thereon are press fitted in the hole in which the spiral groove 1d is formed. Accompanying the press fitting of the bearing 2, the ridge top portion of the groove is crushed while being closely fitted to the bearing 2. However, the spiral groove 1d is not blocked. The ridge portion 1e of the spiral groove 1d is formed into a labyrinth structure functioning as a seal.