The object of the invention is to manufacture a fixed volute element at low cost and to facilitate the positioning between the fixed volute element and an orbital volute element. A fixed volute member 32 includes an end plate 32A and a support member 32B which are manufactured separately. The end plate 32A includes an inner end plate 32Aa, a spiral overlap 32Ab, and a group of cooling fins 52 which is formed on a rear surface 33 of the inner end plate 32Aa. Support member 32B has a circular hole 32Ba in its center into which inner end plate 32Aa is inserted, and housings 39 which house pin crank mechanisms 40 are integrally formed with support member 32B.
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