To provide a means where, in a wheel for a vehicle of a casting, without deteriorating its mechanical properties, a rim is thinned, and the lightening of the wheel can be attained.

Provided is a fixture 1 for working the circumferential face of a casting made of a light alloy comprising: a shaft 140 for a hub; a projecting shaft 65 for a bolt hole and a clamping member 81; a ratchet wrench 68 and stoppers 67a, 67b; a vessel 21; and a working material 25. A wheel 22 for a vehicle is assembled into the fixture 1 for working the circumferential face of a casting made of a light alloy, and oscillation is caused in the upper and lower directions, thus the micro-casting defect of the wheel 22 for a vehicle is easily corrected, so as to improve the mechanical properties of the rim 91. Thus, the rim is thinned while maintaining at least the strength of the wheel for a vehicle, and the wheel for a vehicle can be lightened.