PURPOSE: To detect an accurate sum signal by detecting a data part and shaping a high frequency data signal in the data part to generate a sum signal to be a groove crossing signal.

CONSTITUTION: The output of a 2-divided photodetector 6 is inputted to a cumu lative amplifier 7 and a differential amplifier 8, a high frequency component to be a noise is cut out by an LPF 10, the noise-removed signal is converted into a binary signal by a waveform shaping circuit 11, and the binary signal is outputted as a difference signal 16. On the other hand, the output 14 of the cumulative amplifier 7 is converted into a binary groove crossing signal 15 through a sum signal processing circuit 9. The circuit 9 generates the groove crossing signal 15 to be an accurate binary signal from a signal to which a signal recorded in a medium is superposed. Consequently, the groove crossing signal to be used for detecting the relative position relation between a beam spot and the medium 5 and the relative speed of the medium 5 in the radius direction can be detected.