In recent years, with increased size of the manned space program and systems used in the programs, the role of computer simulations has increased. I have long used and developed independently several simulation tools for the design, operation, analysis, and optimization of the Life Support Systems (LSS). I recognized that the designer makes his/her own idea certain while building a simulation model on a computer. However, conventional simulation tools are not designed so that the interaction between the designer and a model building support environment is dynamically used to bring out a designer's idea. Therefore, in this paper, I consider the development of a conceptual design support tool in the design of the LSS, while focusing attention on the interaction between the simulation tool and the designer.