Abstract: In order to improve product quality control through quantitative determination of a defect and quantitative judgement of the quality of a product, an alternating current is applied to a standard product coil into which is inserted a standard object, and to a test object coil into which is inserted a test object, and eddy currents are measured by means of a bridge circuit. If the test object coil is at a location removed from a flaw, the bridge circuit output voltage is zero, while if the test object coil is at the location of the flaw, a predetermined voltage is output. A determination of torsional breakage torque of the shaft inspection is carried out by correcting the output voltage for dependence on the cross sectional area ratio of the test object relating to the cross sectional area of the coil and for the influence of flaw size depending on the location.