PURPOSE: To prevent the breakage caused by the contact with the bulk-head of a magnet, by detecting the attractive force in the axial direction of a magnet through bearings by a piezoelectric element detector and by monitoring the gap between magnets.

CONSTITUTION: A piezoelectric element detector 9 is installed to the magnet side of a bearing box 3. The attractive force in the axial direction of a magnet 4 is detected with this piezoelectric element detector 9 through bearings. After the electric output of the piezoelectric element detector 9 is amplified with an amplifier 10, it is inputted into a programmed controller 11. In comparison between the amplified piezoelectric element signals and preset electric signals if it is found the piezoelectric signals exceed the upper of lower limit, this programmed controller will cut off the feeding to a motor 1.