A single-mode optical fiber has a prescribed mode field diameter (MFD.sub.1 (.mu.m)) at a first wavelength .lamda..sub.1, in which a bending loss when measured at a second wavelength .lamda..sub.2 (.mu.m) and wound with a bending radius r (mm) is L.sub.b (dB) for one bending, a connector/splice loss with an optical fiber that has a prescribed mode field diameter MFD.sub.2 (.mu.m) at the first wavelength .lamda..sub.1 is L.sub.s (dB) for one connection/splice point at the second wavelength .lamda..sub.2 (.mu.m), and an mode field diameter dependence of a total loss coefficient calculated by a formula (1) has a local minimal value in a range of MFD.sub.1.+-.0.5 .mu.m, with the formula (1) being as follows: L=w.sub.sL.sub.s+w.sub.bL.sub.b, (1) w.sub.s+w.sub.b=1, (2) w.sub.s>0, w.sub.b>0, (3) where w.sub.s and w.sub.b in the formula (1) represent dimensionless weighting factors and are set within a range that satisfies the formulas (2) and (3).