The purpose of clinical diagnosis is to determine the most fitting treatment for individual pathological conditions, and the process of diagnosis and treatment is one of feedback control system. In this paper, application of “optimal control theory” to diagnostic process was discussed and one example of computer program was showed. When diagnostic process is regarded as one of control system, medical treatment, pathological internal conditions and clinical findings are replaced to input variables, state variables and output variables respectively, and if the interrelationship of these three variables is quantitatively represented, it becomes to a problem of optimal control theory. As a clinical application, “insulin therapy in diabetic coma” was proposed. Injection of insulin and infusion therapy as input variables, and blood glucose level and insulin level state variables were used respectively, and the iterrelationship was mathematically represented based on the simulation study of blood glucose kinetics. This model was programmed for digital computer. When observed data of blood glucose level and details treatment were entered, the dosis of insulin to be injected now, fitting to the desired blood glucose level after adequate time, is computed.