A compressor rotor 6a and a turbine rotor 7a are respectively attached at either end of the main shaft 13 so that the compressor rotor 6a is driven by the power generated by the turbine rotor 7a. The main shaft 13 is supported by rolling contact bearings 15 and 16 relative to the radial direction. The thrust force applied to the main shaft 13 is supported by electromagnets 17. A sensor 18 is provided to detect the thrust force which affects the main shaft 13 through air within the compressor 6 and the expansion turbine 7. In the sensor 18, sensor elements having the properties changeable in accordance with a pressing force and capable to electrically detect the change in the properties are arranged in a circumferential direction of the main shaft. The sensor 18 detects the thrust force from the output of the sensor elements.