Abstract: In an apparatus for detecting a focusing error of an objective lens, a light beam reflected from an optical disk passes the objective lens and is reflected from a beam splitter. The reflected light beam is incident on a cylindrical lens so that the light beam is converged in one direction by the cylindrical lens. The converged light beam is incident on first and second triangular prisms so that it is divided into two beams and the two beams are guided into different directions and form beam spots on photo-detectors, respectively. In the respective triangular prism, light rays of the light beam pass through different optical paths. Thus, an aberration is applied to the light beam from the prisms. Accordingly, the shape of each beam spot on the photo-detector is changed in accordance with a focusing error.