A multistage pump, comprising a plurality of intermediate casings (2) formed by press-molding a steel plate, the intermediate casing further comprising a cylindrical side face part (21), a stage flat face part (23) coming into contact with the axial end face of the adjacent intermediate casing, a step side face part (24) axially extending from the stage flat face part, and a bottom face part (25) extending from the step side face part in radial inner direction, wherein a relief plate (30) having an outer peripheral end face coming into contact with the inner peripheral surface of the cylindrical side face part of the adjacent intermediate casing is installed at the bottom face part of the intermediate casing so that a space for installing an O-ring (40) can be formed of the relief plate, stage side face part, stage flat face part and the inner peripheral surface of the cylindrical side face part of the intermediate casing.