Abstract: A casing shaped from a light metal material by diecasting, the casing having formed at its inner wall surface a mounting portion having a circular inner surface, the mounting portion being adapted to receive a bearing member having a circular outer surface and serving to journal a shaft. The circular inner surface of the mounting portion has formed thereon a plurality of supporting protrusions spaced at predetermined intervals in the circumferential direction, and the bearing member is press-fitted into the mounting portion. Alternatively, the mounting portion into which the bearing member is to be press-fitted is defined by an annular protruding portion having formed therein a plurality of slits spaced at predetermined intervals in the circumferential direction.