1. A photosensor system comprising: a photosensor array constituted by two-dimensionally arraying a plurality of photosensors; an image reading section which reads a subject image at a predetermined image reading sensitivity by using the photosensor array; a sensitivity adjustment image reading means for reading a sensitivity adjustment subject image while the image reading sensitivity in the photosensor array is changed to a plurality of stages; an optimal image reading sensitivity deriving means for deriving an image reading sensitivity optimal for reading operation of the subject image, on the basis of a pixel data group relating to an image pattern of the subject image read by the sensitivity adjustment image reading means; and an image reading sensitivity setting means for setting the optimal image reading sensitivity as the image reading sensitivity in the image reading section.