PURPOSE: To detect a speed with a high resolution by obtaining the spot position from the analog distribution of the reflected light intensity signals and the pattern of access code and obtaining the amount of spot motion and speed by the difference with respect to the previous sample.

CONSTITUTION: Sampled results of RF sum signals, which represent the reflected light intensity in the access code region, are stored in a memory 7 and are compared with an access code pattern of a pattern storage means. The result of this comparision is inputted to a position computing section 8 to compute the position of the light spot and the result is stored in a memory 9. Thus, the output of the memory 9 and spot position outputs by the position comparing section 8 at different times are inputted to a subtracting section 11 to perform a subtaraction. Then, the output of the subtracting section 11 is multiplied with a constant by a multiplier 12 to compute a crossing speed of the light spot.