To provide a steam turbine 20, provided with a rotor 21 provided with a number of rotating vanes 22 disposed inside a casing jacket 30 comprising a number of casing segments with a number of guide vanes 24, which is designed to be suitable to a relatively high steam parameter, that is specially high steam temperature, and operation under specially high steam pressure, and an operation method thereof.

For positively cooling the casing jacket 30, at least one of the casing segments forming the casing jacket is provided with a number of built-in cooling passages 29. Consequently, the casing jacket 30 is positively cooled by energization of the cooling passages 29 by cooling medium when the steam turbine 20 is operated.