An image forming apparatus using electrostatic images includes a state quantity detection device for detecting states which would exert some influence on the formation of images as quantities, a control quantity control device for controlling the operation of a process for forming images on an image bearing member, a rule storage device for relating the relation between the state quantities and the control quantity by a control device as a certain rule and storing it, and an inference device for inferring a control quantity to be determined from a set of state quantities on the basis of rules of the rule storage device. The picture image forming apparatus determines the operation quantity for the image bearing member of the process device on the basis of the calculated results of the inference device and forms an image.