PURPOSE: To realize control by considering the characteristic of a product or the using characteristic of a customer by providing an automatic adjustment means for a membership function automatically adjusting a membership function for potential control.

CONSTITUTION: In a main control part 10, a control program storage part 2 functions as a program storage means storing a program for control and an inference regulation storage means storing an inference regulation for establishing relation between state quantum and manipulated variable as a qualitative regulation. A data storage part 3 functions as an information storage means storing controlled variable, the state quantum, the manipulated variable and arithmetic data and a function storage means storing such a function that the state quantum and the manipulated variable are expressed by at least one membership function. Besides, an arithmetic processing unit(CPU) 1 functions as an arithmetic processing means executing arithmetic processing by the information stored in the respective storage means and controlling a controlled system by a fuzzy inference. Therefore, the surface potential of a photoreceptor can be controlled by the fuzzy inference more suitable for each product.