For an index change of an interfacial region 20 which is formed in a boundary between core region 10 and cladding region 30 and which decreases its refractive indices from the core region 10 side to the cladding region 30 side, a change rate in relative index difference (0.5×&Dgr;n)/(d/r) normalized by a core radius r, obtained for a domain of change from 0.8×&Dgr;n to 0.3×&Dgr;n relative to a relative index difference &Dgr;n of the core region 10, is used as an index for evaluation of refractive index change and an allowable range of values thereof is set to a range of the lower limit of 0.4% to the upper limit of 4.0%, whereby a single-mode optical fiber capable of low loss phototransmission can be realized while reducing influence of a tail spread and strain in the optical fiber.