A steam turbine comprises, in combination, at least two of a high pressure turbine section, an intermediate pressure turbine section and a low pressure turbine section in a single turbine casing and the steam turbine generally satisfies such design requirements as: a main steam pressure of 100 kg/cm2 or more; a main steam temperature of 500° C. or more; a rated output (power) of 100 MW or more; and a unit rotated at a rotation speed of 3,000 rpm equipped with a last-stage movable blade of the turbine having an effective blade length of 36 inches or more, or a unit rotated at a rotation speed of 3,600 rpm equipped with a last-stage movable blade of the turbine having an effective blade length of 33.5 inches or more. In such steam turbine, a turbine exhaust chamber of the low pressure turbine section has a structure extending towards both sides of a transverse direction of the turbine casing, towards the upper side thereof or in the axial direction thereof.