Abstract: According to the present invention, a concept of functional quantities indicative of quantified functional degrees of such lower-level functions as "clearer characters", "cleaner background", and "thicker characters", which are subordinating to a qualification-oriented function of "more beautiful characters". Characteristic patterns of functional quantities F1, F2 and F3 of the lower-level functions for physical parameters (for example, the surface potential of a photoreceptor body) are prepared. When a fault repair is implemented by changing a value of a physical parameter, a tradeoff occurs between lower-level functions which are influenced by the physical parameter. That is, taking into account the functional quantities F1, F2 and F3 of the lower-level functions, the value of the physical parameter is set so that the functional quantity of the function of "more beautiful characters" is maximized.