PURPOSE: To prevent the generation of offset by temporarily adjusting the position of a sensor when an information recording carrier for adjustment is used, successively measuring the quantity of stray light in this state and also calculating the superimposed quantity of the offset.

CONSTITUTION: The superimposed quantity of offset V0 is the offset quantity of tracking error signals TE when the information recording carrier 7 for adjustment is brought in, and is the sum of sensor signal quantity (X.R1/R2) and stray light quantity (-X). While the sensor signal quantity is influenced by the reflectivity of the information recording carrier, the stray light quantity is not. Consequently, out of the offset quantity of the tracking error signals when the information recording carrier is actually used, the sensor signal quantity is (X.R1/R2).(R2/R1)=X, and the stray light quantity is not changed but stays as (-X). Thus, when the information recording carrier is actually used, the offset quantity of the tracking error signals is X-X=0.