To enhance a stress-resistance of the outermost periphery of an impeller and to make a disc part thinner.

A disc part 4 of the impeller 1 is formed such that it is made to a cross section shape continued from a hub part 3 and an axial thickness is gradually reduced accompanying with a radial and outward extending. A part extending from an intermediate part to near the outermost periphery in a radial direction is made as thin as possible. A reinforcing ring part 6 made to be thicker at a back surface opposite to a blade part 5 and circumferentially continued is formed on the outermost periphery of the disc part 4. A stress at an impeller outlet can be reduced by making a thickness of the outermost periphery of the disc part thicker and enhancing a rigidity at this part and an influence caused by a centrifugal force can be reduced by making other parts of the disc part 4 as thin as possible. Therefore, a problem of HCF is solved by a high rigidity at the impeller outlet part and a light weight can be realized by making the thick part to the minimum.