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.