A method comprises a calibration curve creation step in which a calibration curve is created on the basis of hardened surface layer depth. A destructive test is performed on one of two samples in pairs, and propagation time of a first peak of a first wave of waveform of a signal outputted with the other in pairs in a non-destructive test is determined, the samples in pairs being prepared such that carburization depth varies gradually from pair to pair. A waveform of a signal outputted with a to-be-inspected piece is obtained. The propagation time of a first peak of a first wave of the waveform of the signal output with the to-be-inspected piece is obtained, and the hardened surface layer depth of the to-be-inspected piece is obtained using the calibration curve, on the basis of the propagation time obtained with the to-be-inspected sample.