According to one embodiment, modulation processing to convert digital information sequences into a form that meets a request from a recording/reproducing system is performed on a digital information sequence and a dummy bit is added to the sequence, and furthermore, an error correcting parity bit sequence is added to the digital information sequence. If the added parity bit sequence does not correspond to a form that meets a request from the recording/reproducing system, a value of the dummy bit is changed and the parity bit sequence is replaced with a parity bit sequence that corresponds to the dummy bit of the changed value.