Abstract: A plurality of devices for processing IF-THEN JOIN operation of IF-THEN rules are provided to process the JOIN operation in a pipe-line manner. A JOIN operation processing facility has an input buffer for latching status data corresponding to one condition in a certain rule; an associative memory for latching status data corresponding to another condition; an output buffer for latching the result of a JOIN operation; and a data combining arrangement for replacing an item value from the two data by a meaningful value to generate one data so that a series of operations of picking up the data one by one from the input buffer, retrieving associatively the data of the associative memory by using the data picked up as a retrieval data and the common item as a retrieval key, and storing in the output buffer the combined result of the data picked up and the retrieval data.