PURPOSE: To facilitate assembling by securing a rotor disc and a rotary encoder disc, while adjoining each other, to a rotary shaft.

CONSTITUTION: A rotary shaft 5 is supported by upper and lower bearings 3, 2 and a rotor supporting section 5a having a large diameter is formed in the center of the rotary shaft 5. A winding rotor disc 6 and a rotary encoder disc 7 having larger radius are secured, while adjoining each other, to the rotor supporting section 5. The rotary encoder disc 7 is made of a plate glass where a pattern 7a is formed at a section having larger radius than the winding rotor 6, i.e., the outer circumferential section. The pattern splits the circumference (360°) into about 21,600 segments and the pattern is arranged only in the range of 60°-90° in which the rotary shaft 5 actually rotates.