Present invention relates to a phase difference detection type focus detecting device which includes a pair of image forming lenses and a photo sensor array and detects the focus deviation in accordance with the positions of a pair of formed images respectively formed on the photo sensor array by the pair of image forming lenses. In such focus detecting device, the pair of image forming lenses are integrally molded of a plastic material, such that the change of the distance between the pair of image forming lenses due to the temperature change causes the error of the focus deviation. The error is made to zero by the detection of the ambient temperature and the compensation of the data basing on expansion or contraction of the plastic material with temperature.