1. A fuzzy membership function operating device for determining a control value for a controlled object, comprising: membership function storage means for storing membership functions; inference rule storage means for storing fuzzy inference rules; parameter storage means for storing parameters required to alter said membership functions; target value storage means for storing a target inference value and a permissible range of said target inference value; inference rule management means for fetching corresponding membership functions and inference rules from said membership function storage means and said inference rule storage means; fuzzy inference operation means for making fuzzy inference of said control value and operating on an inference result using said inference rules and membership functions; target value/result value comparison means for comparing said target inference value and inference result value; and membership function control means for receiving parameters from said parameter storage means and altering said membership function according to said parameters if said inference result does not satisfy said permissible range of the target inference value, wherein said parameters comprise increments of an alteration.