The importance of developing a support system for medical decision-making in the clinical field has been pointed out for improving accuracy and objectivity of the Judgment of clinician, and for detecting the early disease.

In this paper, noticing that signs, symptoms, clinical laboratory data are essential information which show functional depression and failure of the ecosystem, and a practical software system is providing the physicians with clues to clinical diagnosis. First, by analysis of the medical task and structuralization of patient information, the medical modelling method is provided, where diagnosis can be considered the identification of patient who corresponds to the controlled object. Secondly, by the concept of patient model and disease model, the system identification algorithm is proposed, and actual system constructed for the medical diagnosis is described to confirm the usefulness of the proposed method.