Provided are an optical transmission system in which one can easily set a video signal cable, and an adapter that can be used in this system. There is provided an adapter for use in an optical transmission system in which an analog video signal or a digital video signal is transmitted between a pair of adapters using light. The adapter includes a first signal determination unit configured to make a determination as to whether a first electrical signal inputted to a common input terminal is an analog video signal or a digital video signal.