1. A computer system having an arithmetic and logic device for processing, in accordance with a given instruction, data words serving as input data and output data, the data words each being divided into an element field with data representative of an element of a fuzzy set and a grade field with data representative of a grade corresponding to the element represented in the element field, said system comprising: an element operating unit for processing element field data, a grade operating unit for processing grade field data, an instruction decoder for interpreting a given instruction to supply a first instruction to said element operating unit and a second instruction to said grade operating unit, input registers for supplying element field data of input data to said element operating unit and grade field data of input data to said grade operating unit; and an output register for composing outputs from said element operating unit and said grade operating unit into an output data word, and for outputting the output data word as output data.