PURPOSE: To aim at low-unit cost of production by clamping a lower housing to the lower part of an upper housing rotor, fitting a steel ball in a recess installed in the center of this lower housing, and pressing an lower end of a hollow shaft to this steel ball free of rotation.

CONSTITUTION: An upper housing 6 is unitized with a rotor 5 as one body, and a lower housing 9 is attached to a boss 10. A recess 9 to be fitted with a steel ball 7 is formed in the center of the lower housing 9, making it press a lower part inner diameter 1a of a shaft 1 and an outer circumference of the steel ball 7. Accordingly, an inexpensive bearing part can be constituted owing to the steel ball, thus low-unit cost of production is contrived.