PURPOSE: To enable to change a transmission line for making a light beam branch or combining it without causing a loss of a light beam, by coupling in one body two or more films having the complementary wave length selectivily, and inserting them into the opposed part of the optical transmission line so that they can be moved.

CONSTITUTION: Films 11, 12 whose length selectivity is complementary are provided on the inside of the light transmission substances 13, 14 such as glass, etc., by means of evaporation, etc. The films 11, 12 are provided on the same plane so that they are not superposed on each other. The film 11 makes a light beam of λ1 transmit and reflects that of λ2. The film 12 makes a light beam of λ2 transmit and reflects that of λ1. When light beams having the wavelength of λ1, λ2 have been made incident from the optical fiber 15a, they become a parallel flux of light by the lens 16a, and a light beam of λ1 and that of λ2 are made incident to the optical fiber 15b and the optical fiber 15c, respectively, through the film 11. On the contrary, when light beams of λ1 and λ2 have been projected from the optical fibers 15b, 15c, respectively, they are made incident to the optical fiber 15a. When the film 12 is moved so as to come to the position of the film 11, the transmission line can be changed without causing a loss of a light beam.