PURPOSE: To perform defocusing detection with high precision over a wide range and to obtain a tracking deviation signal precisely by processing output signals of two-division photodetecting elements provided across a specific optical system in directions orthogonal to an optical axis and a four-division photodetecting element provided on an optical axis.

CONSTITUTION: Light emitted by a semiconductor laser 1 is converged on a disk 6 through a polarization beam splitter (PBS)2, λ/4 plate 3, collimator lens 4, and objective lens 5. Its reflected light is passed through the PBS2 and split in three directions by a beam splitter 7 to enter the four-division photodetecting element 10 through the two-division photodetecting elements 8 and 8' at image non-formation points B and B' at equal short or long distance from the conjugate points A and A' of the laser 1 and a cylindrical lens 9 on the optical axis. A defocusing signal is obtained from the sum signal of difference components obtained by subtracting outputs of the photodetecting elements 8 and 8' from the output of the photodetecting element 10 and the sum signal of difference signal of the diagonal sums of two couples of photodetecting elements 10 and the tracking deviation signal is obtained from the difference signal of the outputs of photodetecting elements 8 and 8'.