An optical fiber for forming an optical transmission line suitable for the
wavelength division multiplexing transmission by connecting to a positive
dispersion optical fiber is provided. Outside a first glass layer (1), a
second glass layer (2), a third glass layer (3) and a fourth glass layer
(4) are sequentially disposed. The fourth glass layer (4) is to be a
reference layer for a standard of a refractive index. A relative
refractive index difference .DELTA.1 of the first glass layer to the
reference layer is set to 1.6% to 2.6%, inclusive, a relative refractive
index difference .DELTA.2 of the second glass layer is set to-0.65% to
-0.4%, inclusive, and a relative refractive index difference .DELTA.3 of
the third glass layer is set to 0.15% to 0.5%, inclusive.
