to determine whether there is any data defect existent in the data signal in a case that there are serious and small scratches existing in the data storage region of a disc the relationships among sbad rfrp 1 data and defect indicating signals are shown in fig4 a due to a serious scratch in the data storage region of the disc a data signal drop e 11 of the data signal e 1 is rendered thus a signal drop s 11 occurs in the sbad signal s 1 and a signal drop f 11 occurs in the rfrp 1 signal f 1 correspondingly afterwards another scratch that is less serious in the data storage region of the disc is encountered so a less data signal drop e 12 of the data signal e 1 is rendered correspondingly a less signal drop s 12 occurs in the sbad signal s 1 and a less signal drop f 12 occurs in the rfrp 1 signal f 1 in another case that there are data interruptions existing in the data storage region of a disc the relationships among sbad rfrp 1 data and defect indicating signals are shown in fig4 b as shown signal drops f 21 and f 22 of the rfrp 1 signal f 2 occur corresponding to the data interruption e 21 and e 22 and become lower.