PURPOSE: To improve a test efficiency, realize an on-line application and save labor by providing an intake air seal member having a gas intake chamber inside and arranged opposite to a pressing seal member so as to sandwich the weld of the wheel of an automobile between the intake air seal member and the pressing seal member.

CONSTITUTION: A weld defect detector is composed of a frame 10 for holding a wheel 1 on a receiving surface, a pressing seal member 11 provided outside and inside the weld of the wheel 1 below the frame 10 and having a gas pressurizing chamber 11a inside, a supply unit 21 for supplying the pressurizing chamber 11a with a testing gas, an intake air seal member 12 provided opposite to the seal member 11 so as to sandwich the weld between the members 11 and 12 below the receiving surface and having a gas intake chamber 12a inside, a leak detector 26 provided for detecting the leakage of the testing gas from the weld and connected to the gas intake chamber 12a and the like. When the seal members 11 and 12 are raised to the position of the weld and brought into contact with the weld under pressure, the testing gas leaks to the seal member 12 side to be detected by the detector 26 and the defect of the weld can be detected, when the defect is present in the weld.