A steam turbine 10 is comprised of a double-structured casing configured of an outer casing 21 and an inner casing 20, a turbine rotor 23 disposed through the inner casing and having a plurality of stages of moving blades 22 implanted, and a plurality of stages of stationary blades 25 disposed alternately with the moving blades 22 in the axial direction of the turbine rotor 23 in the inner casing 20. The steam turbine 10 is further provided with a discharge passage 30 which externally guides steam, which has flown in the inner casing and passed the final stage moving blades while performing expansion work, directly from the inner casing interior.