An image signal processing apparatus to realize signal processing system and apparatus constitution, where an image signal inputted using a photoelectric conversion element such as a CCD or a contact type image sensor is obtained in circuit constitution of small scale with high picture quality. In order to solve the problems, data width of a signal referred to in a shading corrector, an MTF corrector and an error diffusion circuit is made b + f + j ≦ 8 per one pixel. Or error component commonly possessed by plural pixels (N pixels) in the error diffusion circuit is made b + f + j' ≦ 8 , data width per one pixel being made j' = j/N . Image signal processing is realized in this constitution, thereby data width of a signal referred to in respective signal processings is reduced, and an image signal satisfying accuracy of signal processing sufficiently and having high picture quality can be obtained. For example, when a circuit is constituted by LSI, the memory section can be easily mounted on the same LSI. A memory required in the image signal processing can be made 8 bits per one pixel and a cheap memory constitution can be utilized.