In a steam turbine including a rotor including a free side end fixed by a journal bearing in a radial direction and a fixed side end fixed by a thrust bearing in an axial direction, and a casing including a fixed side end fixed by the thrust bearing in the axial direction, a casing position adjustment device is configured to adjust an axial position of the casing with respect to the rotor due to thermal expansion. The casing position adjustment device includes: a low-pressure casing end plate, which is an end plate oriented to a free side in the axial direction in a low-pressure casing of the casing, and has a diaphragm deformable in the axial direction; and actuators, which are configured to deform the low-pressure casing end plate so that the low-pressure casing end plate extends toward the free side in the axial direction.