A braking mechanism is provided that is suitable for an expansion turbine that rotates at high speed. Upper end salient poles 26a and 26b are formed in two opposite places on the outer peripheral upper end of a rotating shaft 12, and lower end salient poles 28a and 28b are formed in two opposite places on the outer peripheral lower end of the rotating shaft 12 such that they are staggered in the vertical direction with respect to the upper end salient poles 26a and 26b. A casing 22 is provided in a location facing an outer periphery of the rotating shaft 12, and an excitation coil 30 is provided on the casing 22 for forming a magnetic path between the upper end salient poles 26a and 26b and the lower end salient poles 28a and 28b. By rotation of the rotating shaft 12, and by the magnetic path formed by the excitation coil 30, eddy currents are generated in the casing 22.