PURPOSE: To heighten the working efficiency of a disk device by performing an input/output operation on an alternate track or an alternate sector with alternate link information stored in a nonvolatile memory.

CONSTITUTION: When a write or readout operation instruction is issued from a host device (HOST) 12, it is judged whether or not and address designated in the instruction is the address of a defective sector. When the address of the defective sector is designated, the address of the alternate sector for the defective sector is read out from a second read-only memory (ROM II) 8, and a write or readout operation is performed by a magnetic disk device (DKU) 11 by designating the address of the alternate sector. Thereby, the HOST 12 can perform the designation of the address in disregard of the defective state of the DKU 11, which reduces a load on the HOST.