A photosensor system is provided which includes a photosensor array constituted by two-dimensionally arraying a plurality of photosensors, a driver circuit for supplying a drive signal to the photosensors, and a controller for controlling a reading operation of a subject image and a sensitivity setting. Before the start of normal reading operation of a subject image, a pre-reading operation of changing the image reading sensitivity at a plurality of stages for respective rows is executed. A row in an optimal image reading state is easily determined based on the dynamic range distribution of the lightness data of read image data or a dynamic range distribution from which an abnormal value deviating from the main change trend of lightness data is removed, and the linearly differentiated value of the dynamic range. An image reading sensitivity set for this row is set as an optimal sensitivity.