A focus detecting device is provided in which at least a line sensor having a plurality of picture elements is used for receiving light having passed through an objective lens to produce image data. The image data is compared with data shifting the image data by a unit of one or more picture elements to calculate the difference therebetween. It is then determined whether this difference exceeds a predetermined range. Various alternative scheme of calculations on the image data, delayed image data, and differences thereof are disclosed and are responsive to an indication of whether the predetermined condition is exceeded, for identifying defocus of the objective lens. These schemes include one in which, when the difference exceeds the predetermined range, the difference data are reduced by a predetermined rate for calculating defocus of the objective lens.