PROLEG: An Implementation of the Presupposed Ultimate Fact Theory of Japanese Civil Code by PROLOG Technology. In this paper, we propose a legal reasoning system called PROLEG (PROlog based LEGal reasoning support system) based on the Japanese “theory of presupposed ultimate facts” (called “Yoken-jijitsu-ron” in Japanese, the JUF theory, in short). The theory is used for decision making by judges under incomplete information. Previously, we proposed a translation of the theory into logic programming. However, it turns out that the knowledge representation in logic programming is difficult for lawyers to understand. So, in this paper, we change knowledge representation of rules in the JUF theory in PROLEG so that we reflect lawyers’ reasoning using the idea of “openness” proposed by a judge who is a main investigator of the JUF theory.