To considerably reduce a temperature difference and a pressure difference to be absorbed in a valve housing by providing such a state that a valve casing part for holding flowing-in steam being surrounded by steam to be filled in a space between an inner casing and an outer casing.

A steam turbine is substantially provided with a bladed rotor 2, an outer casing 4, at least one inner casing 3, a raw steam pipe route 8 used for flowing-in steam 13, a valve 22 for setting the quantity of the flowing-in steam 13, and a discharge steam pipe route 19 used for flowing-out steam 17 and the valve 22 is arranged in the connection part 4a of the outer casing 4. A part of a valve casing 10 for allowing the flowing-in steam 13 to enter therein is surrounded by such steam as being filled in a space 18 between the inner casing 3 and the outer casing 4.