A plurality of optical fibers are arranged in parallel and peripheries of
the plurality of these optical fibers are integrally formed using a
sheath. Here, the sheath is formed over the entire length of optical
fiber ribbons and a flat portion of the sheath is formed substantially
parallel to a common tangent of the neighboring optical fibers. A maximum
value of a thickness of the optical fiber ribbons is set to a value which
is larger than an outer diameter of the optical fibers by 40 .mu.m or
less. Due to such a constitution, it is possible to easily branch the
optical fibers from the integrally formed optical fiber ribbons.