PURPOSE: To improve the error rate of recovered data by comparing a sum between an equalization signal being an output of an identified point and a path metric difference corresponding to likelihood difference subject to one-bit delay with a preset level so as to obtain detection data.

CONSTITUTION: A reproduced signal from an optical memory or a magneto- optical memory or the like passes through an equalizer 1, in which the signal is waveform-equalized and the equalization signal is subject to PLL control by a PLL 2 and a recovered clock signal is generated. Furthermore, the equalization signal is given to a computing element 3 including a comparator, where the signal is compared with a level 1 and a level 2 and a detected data output OUT0 and an output OUT1 are outputted. Then the detected data OUT0 and OUT1 are given to an error correction discrimination circuit 4 being a decoding value estimate means as tentative detection data and subject to error correction by the maximum likelihood decoding method and the result is outputted as detection data.