PURPOSE: To evaluate the strength of a spot welded section of a wheel by measuring the deformation of the back of the spot welded section of a rim with a distortion gage when a rim side jig and a disc side jig are pulled in the direction opposite to each other.

CONSTITUTION: A concave 13 is formed on a substrate 10 of a rim side jig 8 at the position just corresponding to a spot welded section 4 so that the circumference of the rim is exposed without contacting the substrate 10 where the concave 13 is positioned. A strain gage 14 is applied on the exposed surface thereof and a lead wire is led to a measuring apparatus. Under such a condition, pulling the jig 8 and a disc side jig 9 also causes the rim to be deformed. A distortion due to the deformation is measured with the gage 14. In this case, the distortion is small when a sufficient nugget is formed but large otherwise. This enables the evaluation of the strength of the welded section.