A single-mode transmission optical fiber includes a central core region radially outwardly from a centerline to a radius r1 and having a positive relative refractive index &Dgr;1; a first inner cladding region extending radially outwardly from the central core to a radius r2 and having a negative relative refractive index &Dgr;2; a second inner cladding region extending radially outwardly from the first inner cladding region to a radius r3 and having a non-negative relative refractive index &Dgr;3; an intermediate cladding region extending radially outwardly from the second inner cladding region to a radius r4 having a negative relative refractive index &Dgr;4 larger in absolute value than the relative refractive index &Dgr;2; and an outer cladding region extending radially outwardly from the intermediate cladding region and having a non-negative relative refractive index &Dgr;5; wherein the relative refractive index &Dgr;2 of the first inner cladding region is −0.1·10−3 to −1.0·10−3 and the relative refractive index &Dgr;4 of the intermediate cladding is −3.0·10−3 to −5.0·10−3.