A steam turbine 10 according to an embodiment includes: rotor blade cascades each made up at a turbine rotor 22; an inner casing 21 where the turbine rotor 22 is provided to penetrate; an outer casing 20 surrounding the inner casing 21; stationary blade cascades each made up at an inner side of the inner casing 21; and an annular diffuser 60 provided at a downstream side of a final turbine stage, formed by a steam guide 40 and a bearing cone 50, and discharging steam toward outside in a radial direction. An enlarged inclination angle θ1 of an inner surface 70 of a diaphragm outer ring 26a at the final turbine stage relative to a turbine rotor axial direction is an enlarged inclination angle θ2 of an inner surface 80 at an inlet of the steam guide 40 relative to the turbine rotor axial direction or more.