A stator 6 includes an annular inner shell 11, an annular outer shell 12 and blades 13. The outer shell 12 is arranged radially outside the inner shell 11. The blades 13 are formed between the inner shell 11 and the outer shell 12, and are arranged at a plurality of circumferentially aligned positions, respectively. Each blade 13 extends from the inner shell 11 to the outer shell 12 and thereby couples the outer peripheral portion of the inner shell 11 and the inner peripheral portion of the outer shell 12 together. A rate of change in cross sectional area of the blade changes at a position located radially outside the inner shell 11 at a distance in a range from 1/6 to 1/2 of a distance (r2-r1) between the outer peripheral portion of the inner shell 11 and the inner peripheral portion of the outer shell 12.