A feedback loop structure including an operational amplifier unit (1) for amplifying a reproduction signal of recorded information and a gain/offset control unit (6) for controlling an amplitude and an offset of an output of the operational amplifier unit (1) so that each of the amplitude and the offset becomes a predetermined value is adopted, and accurate information for an asymmetry amount is obtained by a binarization unit (7) for receiving the output of the operational amplifier unit (1) and performing binarization while adjusting a slice level by feedback control so that a duty ratio after the binarization substantially becomes a predetermined value. Based on the asymmetry amount information, a judgment level of a Viterbi decoding unit (15) is changed, thereby reducing a reproduction error rate.