PURPOSE: To provide a device being a storage storing the digital data on a plural tracks memory medium, capable of properly protecting by coding and decoding complexness of a proper extent and realizing such a protection by a fixed format.

CONSTITUTION: These devices are related to the storage and a decoder used together with the medium having plural parallel memory tracks. Error protection is provided by product codes of a C1 code word and a C2 code word. The C1 code word is allocated to a piece of memory segment in respective a piece of memory track. The C2 code word is repeatedly cyclic ranging to a whole tracks, and traverses even a boundary between segments. Physical space between symbols of the C2 code word becomes nearly uniform in the both coordinate axial directions. Further, required memory conditions for decoding are minimized.