The present invention aims to provide a reproduced signal evaluation method and a write adjustment method for offering a Blu-ray disc having a large storage capacity with excellent media compatibility. An evaluation index L-SEAT is calculated through signed addition using a Euclidean distance difference calculated from at least one of target signals in which a focused edge is shifted to the rightand left, and the quality of the reproduced signal is evaluated based on the evaluation index. Write condition adjustment using the index enables write adjustment not depending on SNR and achieving high adjustment accuracy.