PURPOSE: To realize formation of a strong rotational magnetic field and then ensure an accurate detection for the intensity change of the rotational magnetic field with an electromagnetic detector, by mounting a permanent magnet into the nut which fastenes the compressor wing wheel in the exhaust turbo-charger.

CONSTITUTION: Rotating shaft 8 is provided in housing 6 of exhaust turbo-charger 5, and compressor wing wheel 9 is fastened to one end of shaft 8 via nut 10 screwed to the end part of shaft 8. And exhaust turbine wing wheel 11 is fixed to the other end of shaft 8. When engine 1 is working, the air received the pressure increase by the rotational motion of wheel 9 is supplied into the cylinder via conduit 13. And the exhaust gas is discharged into the air after giving the rotational force to wheel 11. Circular groove 25 is formed coaxially with screw hole 24 and in the front of nut main body 23, and then hollow cylindrical permanent magnet 26 is set into groove 25. Thus the rotational magnetic field is formed around nut 10 with rotation of shaft 8, and the intensity of the magnetic field is detected by electromagnetic detector 29.