PURPOSE: To reduce a thrust load in an expansion wherein a body bearing is used by communicating a back space of an impeller with a circulation passage of a braking fan, always equalizing the pressure and arranging both ends of a rotary shaft under the same pressure.

CONSTITUTION: An expansion turbine 1 has an impeller 21 and a braking fan 22 on both ends of a rotary shaft 2 supported within a casing 3. The impeller 21 is rotated by thermal expansion of helium H supplied from a helium compressor 7. Moving helium gas H' is circulated inside a circulation passage 4 accompanied with the rotation of the braking fan 22. The circulation moving rate is adjusted by a control valve 41 for braking the rotation of the impeller 21. In such a constitution, a back space 30 of the impeller is communicated with the circulation passage 4 through a communication lining 6 arranged outside the casing 3. Both ends of the rotary shaft 2 are arranged under the same pressure, thereby a thrust load is reduced.