PURPOSE: To attain the high speed of a fuzzy inference, and the high cost performance of a system.

CONSTITUTION: A fuzzy inference system is constituted of a main controller 30 and a fuzzy inference device 32. The fuzzy inference device 32 is equipped with an interface 32a which receives fuzzy knowledge(a membership function and a fuzzy inference rule) stored in the ROM 30b or RAM 30c of the main controller 30, RAM 32b which stores the received fuzzy knowledge, ROM 32c which stores a control program, and CPU 32d which operates the fuzzy inference. The fuzzy inference of an input value from the main controller 30 is operated by using the fuzzy knowledge, and the result is outputted to the main controller 30. And also, inspection data are stored in the fuzzy inference device 32, an error is detected by comparing it with an inferred result, the RAM 32b is initialized, and the normal fuzzy knowledge is reset.