PURPOSE: To measure deflection amount efficiently over a wide region of an original disc and to detect the abnormality of deflection characteristics in real time by detecting abnormality through comparison of the deflection amount and a predetermined reference value.

CONSTITUTION: A voltage reference 1 derived from a corresponding correction data and a prescribed deflection amount of CD-R is compared with a deflection amount fed from a subtracting section 21 and when the deflection amount exceeds the voltage reference 1, an abnormality detection signal is delivered from a first level detector 25 to a system controller. A second level detector 27 receives a deflection frequency from an F/V converter 23 and compares a voltage reference 2 predetermined based on a prescribed deflection frequency with a deflection frequency voltage fed from the F/V converter. When the deflection frequency exceeds the voltage reference 2, an abnormality signal is delivered from the second level detector 27 to the system controller.