To permit simple assembly through decreasing the number of bearings to one and reduce the product costs.

A yoke assembly 15 is structured so that an electromagnetic coil 14 is installed inside a yoke 12 and a sleeve 30 formed cylindrically from a magnetic substance is inserted in a cylindrical part of a bobbin 26. A rotary shaft 16 is inserted rotatably in the sleeve 30 with the aid of bearings 27 and 29 of synthetic resin installed at the two ends of the sleeve 30. A rotor 18 is fitted on the shaft 16 in such a way as rotatable integrally therewith 16. A hub 40 is rotatably fitted on rotary shaft 16 on the side opposite the coil 14 with the rotor 18 interposed. An armature 42 is installed on the hub 40 side face confronting the rotor 18. Among the bearings, the first bearing 27 installed at the sleeve 30 end on its side with rotor 18 is coupled integrally with that opening edge of the bobbin 26 cylindrical part on the side with the rotor 18.