To inspect defect of a substrate on which a preformat (previously transferred rugged pit information) is formed, such as a stamper and a resin substrate, etc., of an optical disk.

This method is to inspect defect of a substrate provided with preformat information consisting of rugged pits, and from this reproducing signal, the substrate is reproduced by a reproducing device, and a masking signal (c) corresponding to a preformat signal (a) is generated, while a delayed reproducing signal is generated by delaying the reproducing signal, and then this delayed reproducing signal is compared with the masking signal (c) so as to cut a preformat signal (a') of the delayed reproducing signal, thus detecting a defective signal (b).