A magneto-optic disk apparatus is disclosed which can reproduce data well from a magneto-optic disk on which the data are reproduced in a high density with an increased line density. Recording data are converted into NRZI data by a pre-encoder and recorded onto a magneto-optic disk using a magnetic field modulating method while irradiating a laser beam intermittently upon the disk. A reproduced radio frequency signal from the disk is clamped for each segment by a clamp circuit, converted into a digital signal by an analog to digital converter, waveform equalized by an equalizer so that it may be approximated to the waveform of a cosine filter suitable for partial response PR(1, 1), and supplied to a data detection circuit. The data detection circuit detects data using the viterbi algorithm to obtain reproduced data in the form of NRZ data. Since the partial response PR(1, 1) in which inter symbol interference is used positively is applied, even if the line density is raised to achieve high density recording, a sufficient phase margin can be assured. Since viterbi decoding is applied, the S/N ratio is enhanced to reduce the code error rate.