A steam turbine comprises a rotor with moving blades attached thereto; diaphragms which surround the rotor from an outer periphery side of the rotor; a casing which encloses the diaphragms and the rotor and has an upper half and a lower half clamped together through respective flanges; a displacement detector for measuring a difference d in thermal expansion in the rotor axis direction between the casing and the rotor; heating/cooling devices attached to the flanges respectively to heat and cool the flanges; and a controller which makes control so that the flanges are heated or cooled by the heating/cooling devices until a measured value obtained by the displacement detector reaches a preset value M or S in unsteady operation.