PURPOSE: To assure exact writing action and to shorten the waiting time for rotation by executing seeking to a track where the inversion time of magnetic fields is assured up to just prior to arrival of a pickup at a sector to be written.

CONSTITUTION: A PLL circuit 16 forms a pit synchronizing signal by the binarization signal outputted from a shaper 14. A demodulation circuit 17 demodulates the data synchronizing the pits from a discriminator 15 to the same data (write data + synchronizing signal, code for resynchronization + ECC code) written on a magneto-optical disk. An error correction circuit 18 executes error correction(ECC) and forms read data by synchronizing byte from the data demodulated by the demodulation circuit 17. The data is transferred via a host I/f circuit 20 according to the command from a host computer.