A steam turbine 10 in an embodiment includes: an inner casing 21 surrounding a turbine rotor; an outer casing 20 composed of an upper half side outer casing 20a and a lower half side outer casing 20b; and an annular diffuser 50 through which steam passed through a turbine stage is discharged radially outward. A vertical distance H from a axis of a turbine rotor 23 to an inner wall of the upper half side outer casing 20a, an outermost diameter D of final stage rotor blades 22a, a blade height B of the fmal stage rotor blade 22a, and a distance W between inner walls in vertical and horizontal directions to the axis of the turbine rotor 23, at a bottom portion of the lower half side outer casing 20b forming a discharge port 60, satisfy (H - D/2)/B≤1.7 and (W - D)/2B≥2.