PURPOSE: To provide an expansion turbine with a high reliability by reducing a thrust load which is added to the rotating shaft at the time of the-normal operation and transient stage (at the time of start or stop, etc.).

CONSTITUTION: A rotating shaft 2 is internally fitted in a casing 3, and an impeller 21 and damping fan 22 are provided at both ends of the rotating shaft 2. The impeller 21 is rotated by the work done by expansion of helium H which is fed to the impeller 21 from a helium compressor 7, and the damping fan 22 being linked to the impeller 21 circulates a gas H for damping which is sealed in an external circulation path 40, within the circulation path 40. Then, the rotation of the impeller 21 is damped by adjusting the circulation quantity of the gas H' by controlling the opening degree of a speed control valve 41 which is provided on the circulation path 40, as the constitution of the title expansion turbine equipped with a damping fan. For such an expansion turbine equipped with a damping fan, a helium compressor 7 communicates with the circulation path 40 through a first equalizing valve 61.