The focus detecting apparatus comprises an objective lens, a point light source which irradiates illumination light for generating a focusing signal to a transparent substrate through the objective lens, a mask means having a first shading part for shading one of areas of the luminous of the illumination light, and a photodetector having two light receiving parts, wherein the mask means is formed so as to have a shape similar to one of light receiving parts in the photodetector, and has a second shading part that intercepts a part of luminous flux passing through another area so that reflected light from one of the surfaces may enter into the two light receiving parts, and reflected light from another surface may pass through an area which is located off the light receiving part arranged at one of the areas, when a focus of the objective lens is positioned near one of surfaces out of the first or second surface in the transparent substrate.