PURPOSE: To simplify software and to improve reliability by executing a single channel command word at one output of a start I/O instruction to restore or update HA and SD information.

CONSTITUTION: When a defective cylinder is generated in a magnetic disc device and it is necessary to restore HA and SD information to the cylinder, a start I/O instruction is generated from a central processor 1, so that a single channel command word is generated. At the execution of the channel command word, the contents of a recording part 11 in a storage area 10 to be accessed by the data address of said channel command word, an SA cylinder address in a recording part 12 is read out in response to that the contents is restoration control information and a head is positioned on the SA cylinder to read out the HA and SD information. The HA and SD information are stored in a recording part 14. Then, a restoration cylinder address in a recording part 13 is read out, the head is positioned on the cylinder, i.e. the defective cylinder, specified by the address and the HA and SD information read out from the recording part 14 are written in the cylinder.