A data demodulator includes: a conversion means for converting an RLL code obtained by converting data in which information bits including specific bits are inserted at fixed intervals which is included in an input signal in accordance with a modulation table having variable-length conversion rules into data in accordance with a demodulation table corresponding to the modulation table; a determination means for determining control segments for performing calculation intended by the information bits from the converted data; a calculation means for executing calculation intended by the specific bit inserted in the control segment different from a calculation target with respect to the data of the control segment as the calculation target; and a correction output means for selecting one of first data converted by the conversion means and second data obtained by converting the RLL code of the input signal corrected based on the calculation result in accordance with the demodulation table and outputting the data.