A relative refractive index difference .DELTA.1 between a center core region and a cladding layer is 0.30% to 0.35%, a relative refractive index difference .DELTA.2 between an outer core layer and the cladding layer is -0.10% to -0.04%, and .DELTA.1:.DELTA.2 is 2.5:1 to 7.5:1. A diameter of the center core region is 9.0 .mu.m to 10.5 .mu.m, and a ratio of diameters of the center core region and the outer core layer is 0.20 to 0.35. A cutoff wavelength is 1310 nm or shorter, a zero dispersion wavelength is 1285 nm to 1345 nm, and at a wavelength of 1550 nm, an MFD is 10.5 .mu.m or larger, a transmission loss is 0.185 dB/km or lower, and a bending loss is 15 dB/m or lower.