1. A bit converter between an upstream decoder and a downstream decoder, comprising: a substitution module configured to substitute a decision result from the upstream decoder representing a bit string of a predetermined pattern based on a substitution rule for converting the bit string of the predetermined pattern into a predetermined correct code word, the predetermined pattern being used for inspection and having a pattern that cannot be converted into the predetermined correct code word by the downstream decoder; and an output module configured to output the decision result from the upstream decoder as substituted by the substitution module, to the downstream decoder.