To provide a steam turbine installation capable of supplying steam discharged from a high pressure turbine part to other steam handling part and of supplying steam from the steam handling part to a low pressure turbine part.

This steam turbine installation has the high pressure turbine part 9 and the low pressure turbine part 10 connected by a rotor 6. A moisture separator 44 is connected between the high pressure turbine part 9 and the low pressure turbine part 10. A steam shut off valve 50 and a pressure control valve 51 are installed between the moisture separator 44 and the low pressure turbine part 10. An external steam handling part 47 is connected between the moisture separator 44 and the high pressure turbine 9 via a gate valve 46.