The disclosure relates to a focus detecting device for detecting a focusing condition of an objective lens on a focal plane by the detection of contrast of an image formed by the objective lens. The device includes an elongated biconcave cylindrical lens for forming a line image, a glass plate positioned parallelly to the cylindrical lens, and a plurality of photodiodes deposited on the opposite flat surfaces of the glass plate in arrays parallel to the axial direction of the elongated cylindrical lens such that the photodiode arrays on the opposite flat surfaces are disposed alternately in an interlaid manner. When the line image formed on the photodiodes deposited on one surface of the glass plate is front-focus condition and the line image formed on the photodiodes deposited on the other surface of the glass plate is rear-focus condition, a detector indicates that the line image formed on a plane between the opposite flat surfaces is in-focus condition.