One aspect of the present invention provides a steam turbine comprising a rotor and a casing that installs the rotor inside, wherein the casing comprises an upper-half section and a lower-half section, which are coupled with each other by an upper-half flange and a lower-half flange thereof, wherein the upper-half section includes: a semi-cylindrical wrapper plate extending in an axial direction of the rotor, end plates coupled with the wrapper plate, each of the end plate is disposed perpendicular to the rotor, wherein at least one of axial ends of the wrapper plate includes a wrapper plate projecting portion that projects from the corresponding end plates in the axial direction, and a first end plate reinforcing rib disposed on the outside surfaces of the end plates in a peripheral direction of the inner periphery of the wrapper plate; and a plurality of second end plates reinforcing ribs disposed radially on the outside surfaces of the end plates, wherein the second end plates reinforcing ribs couple the wrapper plate projecting portion with the first end plate reinforcing rib.