PURPOSE: To enable a high accurate manufacturing with no attendance of stress generation due to welding, in a manufacturing method of an impeller in a turbo- compressor.

CONSTITUTION: An outer diameter D1 of a main plate material 11 of a main plate is made larger than an outer diameter D0 of an impeller, and is inner diameter d1 is made smaller than an inner diameter d0 of the impeller, and excess thicknesses are given on a height h1 against a height of the axial direction of the impeller, also on the part corresponding to an axis B0 of the outer diameter part. On the other hand, the outer diameter D1, the inner diameter d1, and parts B2, h2 of the side plate material of the side plate have also the excess thickness than the parts of the impeller. And, after being made into a reinfold material which holds a blade width W by means of forming a ring part of the outer diameter D1, an inner diameter D2, and a width B2 in an engaging part h of the side plate material 12, both plate materials 11, 12 are contacted in contacting parts g, h and after being temporally fixed at two locations i, j the side material 12 is combined in one body with a blade 3 by menas of inserting slit by a reverse side bead and thereafter it is finished into a prescribed shape by means of removing the excess thickness.