PURPOSE: To decrease the accessing and to improve the efficiency of use in recording media, by assigning an alternative sector to defective sectors depending on the number of defective sectors on one track or the alternative sectors to all the sectors.

CONSTITUTION: A track sector information bit consists of defective/nondefective track display bit A, defective/nondefective sector display bit B and an original alternative sector display bit C. In initializing the magnetic plane of a rotating type magnetic storage device, the number of defects existing on one track is checked and if the number of defective sectors exists a specified number or over (a number determined with the real storage efficiency of the rotating type magnetic storge device and the degree of frequency of accessing to an alternative sector on a system), this track is taken as a defective track and the alternative track is assigned. If the number of defective sectors existing on one tack is a specified number or below, the alternative sector is respectively assigned to the defective sector.