The present invention concerns a negative-dispersion optical fiber for compensating in a shorter length for chromatic dispersion of a positive-dispersion optical fiber in a signal wavelength band, and an optical transmission line incorporating it. The negative-dispersion optical fiber has the following properties at the wavelength of 1550 nm; chromatic dispersion D of not more than −150 ps/nm/km; a dispersion slope satisfying such a condition that a ratio thereof (S/D) to the chromatic dispersion D is not less than 2.0×10−3/nm nor more than 4.7×1031 3/nm; and an effective area of not less than 12 &mgr;m2 but less than 25 &mgr;m2. For satisfying these properties, the negative-dispersion optical fiber has, in the order stated from the center toward the outer periphery, a core region of a maximum refractive index n1, a first cladding of a refractive index n2 ( n2), and a third cladding of a refractive index n4 (