PURPOSE: To reduce radial thrust and friction loss of a flow passage by arranging tongues and partition walls constituting a volute at equal intervals on the circumference, positioning the end of a partition wall up to the beginning of a succeeding tongue, and specifying the area ratio of an inside flow passage to an outside flow passage partitioned with the terminal end of a partition wall.

CONSTITUTION: A volute casing 201 is provided with (n) over three pieces of tongues 202 equally arranged on the circumference, and respective tongues become partition walls 203 while enlarging the radial positions in the circumferential direction. The partition wall 203 is ended on the position in the circumferential direction from which the succeeding tongue 202 is begun, and the area ratio the inside flow passage 205 to the outside flow passage 206 is instituted nearly as 1:n-1, which are formed on the overlapped part 204 between the terminal end of the partition wall begun fro the (n-1)th tongue and the (n)th tongue 202. By action of the tongue and the partition wall symmetrically positioned, static pressure distribution is made symmetrical against the rotating shaft and hence radial thrust can be reduced. Further, friction loss can be reduced, because the outside flow passage 206 for guiding the fluid flowing along the tongue to a discharge pipe 207 is widened on the lower stream side.