PURPOSE: To facilitate the adjustment and to simplify the constitution by subtracting an output of photodetectors arranged corresponding to a direction orthogonal to an information track to obtain a difference signal, adjusting the mixing ratio of the difference signal and mixing it to form a focus error signal.

CONSTITUTION: In case of obtaining a focus error signal FE from an output of photodetectors 9 arranged with four divisions, outputs of each set of two sets of photodetectors 9 arranged corresponding to the direction orthogonal to the information track are subtracted respectively to obtain two sets of difference signals and the difference signals are mixed by adjusting the mixing ratio to form the focus error signal FE. Thus, it is not required to extract the correction signal from the tracking error signal TE. Thus, the accurate focus error signal is extracted with simple constitution.