Abstract: A focusing state discriminating system of the invention discriminates the focusing state of an image forming optical system, based on signals F1, F2 and F3 representing image formation states of images formed at first, second and third positions of the optical system which substantially correspond to positions slightly in front of, coincident with and slightly behind the predetermined focal plane of the image forming optical system. The operation for discrimination in this system includes at least a comparison of .vertline.F1-F3.vertline. and K(F1+F2+F3), where K is a constant.