To minimize a bit error rate by performing a decision of data detection after adding a prescribed positive offset value to reproduced data before or after a level shifting to make the effect of a Viterbi decoding large.

When a deviation from 'H' to 'L' is to be generated at a level change point where a level is shifted from 'H' to 'L', reproduced data are corrected by preliminarily deciding a prescribed value and by adding the value to the reproduced data. Moreover, when a deviation from 'L' to 'H' is to be generated at a level change point where a level is shifted from 'H' to 'L', reproduced data are corrected by preliminarily deciding a prescribed value and by adding the value to the reproduced data. Consequently, the output of the Viterbi decoding becomes to have no error.