In axial magnetic bearing apparatus in which a rotary disc made of a magnetic material is fixedly attached to a rotating shaft, and electromagnets are disposed on opposite sides of the rotary disc so as to have suitable very small distances therefrom respectively, so as to bear the rotating shaft axially in a non-contact state, a deep groove for forming an air layer having large magneto-resistance is provided all over the outer circumference of the rotary disc. Thus, formation of a magnetic circuit not contributing to position control is relieved, and the weight of the disc is reduced so that the position control performance of the axial magnetic bearing and the natural frequency of a rotor are improved.
CROSS REFERENCE TO RELATED APPLICATION
This is a divisional of Application Ser. No. 10/868,946 filed Jun. 17, 2004now U.S. Pat. No. 7,138,738, which is a divisional of Application Ser. No. 10/089,547 filed Apr. 1, 2002 (issued as U.S. Pat. No. 6,781,269 on Aug. 24, 2004), which is a National Stage Entry of PCT Application No. PCT/JP99/05362 filed Sep. 30, 1999 and is considered part of the disclosure of the accompanying divisional application and are hereby incorporated by reference.
