PURPOSE: To increase the holding force by setting the diameter of a shaft fastening the components constituting a driving magnet train and diameters of center holes of a magnet and a yoke within specific ranges.

CONSTITUTION: The gap between yokes 9 and 18 of a driving magnet train 2 and a driven magnet train 3 is set to about 0.5-1.5mm. The size ratio in the radial direction between an outer magnet 17 and an outer yoke 18 constituting the driven magnet train 3 is set to about 1:0.7-0.95. The size ratios in the axial direction between the outer magnet 17 and the outer yoke 18 and between an inner magnet 8 and the inner yoke 9 are set about 1:0.6-1.0. The diameter of a shaft 12 fastening the components constituting the driving magnet train 2 and diameters of center holes of the inner magnet 8 and the inner yoke 9 are set about 0.2-0.7 of conventional diameters. The holding force can be increased.