The term closed ecological life support system (CELSS) refers to a physical space where energy can go in and out freely but materials cannot do and living things can maintain their lives. A typical example is the Earth. In this huge CELSS, solar heat energy comes in and Earth-generated heat energy goes out but the materials on the Earth cannot go out easily into Space because of the attraction of the Earth, and human beings and other living things can be alive in this closed system. Our research interest is the development of useful techniques necessary in various systems including closed systems; the target covers widely from microbial cells to Space. We are now aggressively working both experimentally and theoretically to the following three systems; 1) metabolic reaction systems, 2) environment-purification systems, and 3) energy-producing systems based on electrolysis. We construct a mathematical model for a given reaction process and carry out a computer simulation to characterize the system and find a better solution.