An optical sensor unit, free from electrical noise, includes a sensor case which has both lateral sides having high frequency connectors mounted thereon, and a metallic plate mounted inside the case. The method then performs bitwise XOR operations between the plurality of groups onto the backup buffer. The FCM signal detection circuit detects an FCM signal based on the laser beam reflected from the track, a comparator compares a level of the FCM signal and a slice level, and a PLL circuit generates a clock signal based on a comparison result by the comparator.