A multistage pump according to the present invention has a plurality of intermediate casings (2) formed by press-forming a steel plate. The intermediate casing has a cylindrical side portion (21), a stage flat portion (23) with which an axial end face of an adjacent intermediate casing is held in contact, a stage side portion (24) extending axially from the stage flat portion, and a bottom portion (25) extending radially inward from the stage side portion. A relief plate (30) having an outer circumferential end face which is held in contact with an inner surface of a cylindrical side portion of the adjacent intermediate casing is attached to the bottom portion of the intermediate casing. The relief plate, the stage side portion, the stage flat portion, and the inner surface of the cylindrical side portion of the adjacent intermediate casing form a space in which an O-ring (40) is fitted.