In a focus detecting device, a plurality of distributions of quantity of light regarding the image of an object to be photographed are formed by optical means disposed on the image plane side of an objective lens by the use of light beams passed through different areas of the pupil of the objective lens. The relative positional relation between the plurality of distributions of quantity of light is found by light receiving means, and the in-focus state of the objective lens is found by the use of a signal from the light receiving means. The optical means has at least three pairs of secondary imaging lenses each forming a pair of light distributions corresponding to the image of the object to be photographed, and is constructed so that at least two of the light segments passing through the vertices of the lens surfaces of the pairs of secondary imaging lenses intersect each other and at least one other light segment does not intersect.