An optical fiber is optically coupled to a light-receiving/emitting element contained in an interface box. The coupling structure is composed of a cylindrical ferule water-tightly inserted into a through-hole formed in a wall of the interface box, a resilient grommet coupled to the ferule and a coupler having the light-receiving/emitting element. An optical fiber is inserted into coaxially formed center holes of the ferule and the grommet, and an axial end of the optical fiber is exposed at an axial end of the ferule. The coupler is coupled to the ferule so that the light-receiving/emitting element directly faces the exposed end of the optical fiber to establish an optical connection at a minimized connection loss.