In this paper, we realize and examine an original chemical structure input system which make us perform input of the synthetic objective, ”target molecule”, which is the first step in computer-assisted synthetic analysis. We use a color graphics display (NEC computer terminal, N6940 color graphics display) to realize this system, and input of the chemical structure is performed by the light-pen. Therefore, it is possible to treat quickly and easily for a person who is unaccustomed to use the teletype of computer.