A focus detecting device having a field lens disposed near the predetermined imaging plane of an objective and a secondary imaging optical system disposed rearwardly of the field lens, whereby a pair of object images based on light beams passing through different portions of the pupil of the objective are formed and the respective object images are detected by photoelectric conversion element rows disposed rearwardly of the secondary imaging optical system to thereby discriminate the focus state of the objective from the amount of relative deviation of the object images is characterized in that a refracting portion for refracting the light beam in the direction of arrangement of the photoelectric conversion elements is provided in the secondary imaging optical system.