<p>This paper presents the architectural overview of the IKD system which functions as an interface between a knowledge-based system (KBS) and a relational database management system. For integrating them more flexibly and naturally at conceptual level, it is necessary to represent conceptual structure provided in both of KBS and relational database (DB). Furthermore, such mechanism are required to be built into a knowledge representation system. The IKD system is a subsystem of the knowledge representation system KBUS which is implemented by us. The IKD system is a KBS for itself, implemented in frame-based knowledge representation. Its knowledgebase includes the description of conceptual data structure of the KBS and DB, and some procedures. From structural viewpoint, this knowledgebase consists of five sub-frame-systems. Among them, two sub-frame-systems play more important role in representing the structures than others. They are data- and problem-structure description sub-frame-systems. A collection of relation schemata is represented in the former, and conceptual objects which the KBS deals with are represented in the latter. Then these sub-frame-systems are related to each other. In addition to them, the knowledgebase has a field description, cursor description, and SQL sub-frame-systems. They represent the data types of the fields in relations, cursors, and components of the database language SQL, respectively. The cursor is specified basing on some problem structure description frame, which manipulate a collection of data stored in the DB by a tuple at a time. While, operations on it are done basing on message passings among those frames. A collection of frame-based data required by the KBS is specified in terms of above described frames. A select-statement to get them is constructed by a sequence of message passings among a problem-and data-structure description frames. A collection of data stored in the DB are captured by sending a message to a cursor description frame. This system is a software tool which is designed basing on a loose-coupling approach for integrating both KBS and DB. Two mechanisms adopted in the system are distinguished as being more effective than others. The first mechanism concerns with the frame, that is, two types of frames are defined ; the one is used for representing a data structure of the DB and the other used for satisfying requirements of realization of the KBS. The second mechanism is devised so that operations on the DB are realized by using several types of message passings. These mechanisms are usable for constructing an interface between the KBS and the DB system. As an application, we tried to integrate a production system and the DB. We describe this application briefly, also. The system is useful in this application system especially for its structural flexibility.</p>