A stable system is provided wherein the influence of disturbance or the like can be reduced. First error correcting code units and second error correcting code units are used, and thereby a wider range of random errors and burst errors are made correctable to avoid retry operation. Thus, stable data reproduction is carried out without degrading transfer rate. Further, the constitution of error correcting blocks is set so that when a head is moved to some track during seek operation, a sector having the second error correcting code recorded therein is the first sector to be read.