PURPOSE:To improve the reliability of data and to shorten the processing time necessary for correcting an error by executing a decision corresponding to the number of errors in respective code strings and a decision as to the normal/ defective condition of a sector data. CONSTITUTION:After a recording data is read out from a medium 101, the valid/invalid-decision of recording is executed in an error correction arithmetic part 105 in a manner as the following. In case the number of correctable error X-pieces of parity data attached to respective code strings in N, an integer Ns less than the N and a calculated number of errors are compared and decided which is larger. If that no errors exceeding Ns-pieces exist in no code strings is confirmed, the total number of errors detected in one sector is calculated. If this total number is confirmed not exceeding the constant number Nt, the normality of the sector is informed to a control part 106, and thereafter ends a RAM check. In case where the total number exceeds the Nt, the error of the sector is informed to the part 106.
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