AbstractThe production system is suited to representing the miscellaneous pieces of human knowledge, and is used most widely in constructing the expert system. A problem is that a tremendous amount of time is required for the condition match, and an efficient condition match algorithm is desired.This paper considers the expert system which has been designed for the inference based on the semantic network, and aims at the realization of the high‐speed condition match in the expert system rewritten by OPS5. The working memory elements which are the objects of investigation are mostly the knowledge of two‐term relations, and the number of attributes usually is 2.This paper also proposes a data structure for the working memory, which can derive directly all working memory elements satisfying the condition, from the condition elements with 2 or less attributes. Using the proposed data structure, an algorithm is shown that can realize a high‐speed condition match. It is shown for the considered expert system that the proposed algorithm can realize a higher speed than the conventional condition match algo