To prevent a high-pressure gas supplied to an expansion turbine from leaking to the outer circumference of the machine to eliminate generation of a fire, high-pressure, gas pollution and over-maintenance caused by foreign matter mixing, and to improve reliability, stability, and economic performance for turbine operation.

In this turbine, a rotary shaft of an expansion turbine 14 into which a high pressure gas like NG is introduced and a rotary shaft of a high- speed generator 13 whose rated speed is adjusted to the expansion turbine 14 are directly connected to be integrated, and a casing 4 of the expansion turbine 14 and housings 3 and 5 of the high-speed generator 13 are connected and are subjected to a hermetic sealing treatment to form a housing integrally so that the turbine has such a sealed structure that the high pressure gas introduced into the expansion turbine 14 is not leaked to the outside of the integrated housing. The bearings which support the shafts installed between the expansion turbine 14 and a rotor 1 of the high-speed generator 13 and the shafts of the rotor 1 extended to an opposite side of the expansion turbine 14, comprise magnetic bearings 7a, 7b, 8a, and 8b.