PURPOSE: To detect focus error, by providing a 1/2 wave length plate having a positive and a negative curvature toward a condenser lens in an optial path of a reflected light of linear polarization and detecting lights through the respective 1/2 wave length plates.

CONSTITUTION: A 1/2 wave length plate 17a has a negative curvature toward a polairzed light beam plitte 12, while a 1/2 wave length plate 17b has a positive curvature. A photodetector 18 is so designed that a light of linear polarization can be transmitted at the maximum strength with a polarized angle by 90° to the light with linear polarization incident to the 1/2 wave length plates 17a and 17b, and a light incident orthogonally to a curved surface of the 1/2 wave length plate best transmits the photodetector 18. Thus, the amount of light received with the photodetectors 19a and 19b is changed when the returned light from the splitter 12 is a diffused light and a condensed light, and from this difference, a focus error detection signal 101 can be obtained.