PURPOSE:To facilitate manufacture by forming an outer plate, installed at the outside of an inner plate so as to surround this inner plate, into a scroll form so as to make a distance from a casing center axis become smaller successively in a range from the inlet side to a roll end part. CONSTITUTION:A section rectangular flow passage is formed by an inner plate 5 erected out of a speed ring 3, and outer plate of constant height H, installed at the outside of this inner plate 5 and so as to surround this inner plate, and an end plate 7 connecting both ends of the inner plate 5 and the outer plate 6. Then, the outer plate 6 is formed into a scroll form so as to make a distance from a casing center axis C become smaller successively in a range from the inlet side to the roll end part. With this constitution, these inner and outer plates 5, 6 are formed into a flat plate form, whereby only the outer plate 6 is good to be formed into a scroll form, so that its manufacture can be done easily.
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