Patent ID: 7648337

Claim:
A multistage pump having a plurality of intermediate casings formed by press-forming a steel plate, characterized in that: each of said intermediate casings has a cylindrical side portion, a protruding portion protruding radially outward from the cylindrical side portion, a stage flat portion extending radially inward from the protruding portion and with which an axial end face of an adjacent intermediate casing is held in contact, a stage side portion extending axially from said stage flat portion, and a bottom portion extending radially inward from said stage side portion, wherein said cylindrical side portion, said protruding portion, said stage flat portion, said stage side portion and said bottom portion are integrally formed from said steel plate by said press-forming, wherein said protruding portion has an outside diameter larger than the outside diameter of the cylindrical side portion, and said stage side portion has an outside diameter smaller than the inside diameter of the cylindrical side portion, wherein a relief plate having an outer circumferential end face which is held in contact with an inner surface of a cylindrical side portion of said adjacent intermediate casing is attached to said bottom portion of said intermediate casing, wherein said relief plate, said stage side portion, said stage flat portion, and said inner surface of said cylindrical side portion of said adjacent intermediate casing form a space in which an O-ring is fitted, wherein said bottom portion functions as a wall separating said intermediate casings.