This steam turbine casing is provided with: a lower half part having a plurality of curved leg parts projecting horizontally around a lower half part body; an upper half part bonded onto the lower half part; a cover member fixed to an upper surface part of the curved leg parts; and curved leg bases each having a lower end part which is placed on a frame and having an upper end part to which a lower surface part of the cover member is fixed, whereby thermal deformation of the casing is suppressed, and a suitable clearance is maintained between the casing and a turbine.