PURPOSE: To design the titled controller so that noise is hardly invaded to a focus error signal even with dirt on the surface of a transparent protective layer of a recording medium by adding an output of a photodetector in phase with respect to a focus error.
CONSTITUTION: A convergent lens 2 converges an irradiated luminous flux 18 onto a recording medium to form a light spot 1. The reflected light of the light spot is collected again by the convergent lens 2, reflectd in a half mirror 11, separated from the irradiated luminous flux 18 and separated spatially into a detection luminous flux 19b through a prism 12 and a detection luminous flux 19a not through the prism 12. The detection luminous flux 19a goes straight and is converged and then made divergent to form an inverted remote visual field on a photodetector 13. On the other hand, the detection luminous flux 19b forms an errected remote visual field image on a the photodetector 13 before it is converged. The focus error detected by the change in each remote visual field image is added substantially to obtain a focus error signal, thereby reducing the effect of dirt or the like on the surface of the transparent protective layer.