PURPOSE: To prevent an increase of the cost of a signal transmission system and to perform multichannel signal transmission by providing plural signal projection terminals which send out optical signals having different wavelength ranges to either of a rotating body and a stationary body facing it.

CONSTITUTION: Either of the rotating body 1 and stationary body 2, i.e. rotating body 1 is provided with plural signal projection terminals 14AW18A which send out optical signals with different wavelength ranges, and the other stationary body 2 is provided with plural signal incidence terminals 14CW18C which have color filters 14BW18B where said optical signals are passed on front surfaces respectively. An optical path 20 surrounded with a reflective surface is formed between those signal incidence and projection terminals, and at least one diffuse transmission plate 22 is so arranged in the optical path 20 as to cross the optical path 20. Wavelength selections are made through the color filters 14BW18B, so only light beams from the signal projection terminals 14AW18A appear at the corresponding signal input terminals 14CW18C.