To obtain a single mode optical waveguide fiber which is easily produced and low in cost.

This single mode optical waveguide fiber is composed of a core region consisting of a central segment, a second segment and a third segment and a clad layer. The max. refractive indices of the central segment, second segment and third segment are respectively n1, n2, n3, the refractive index differences thereof are respectively Δ1%, Δ2%, Δ3% and the radii thereof are respectively r1, r2, r3. The max. refractive index of the clad layer is nC. These refractive indices are n1>n2>n3≥nC. The central segment has an alpha refractive index profile of alpha of about 1, the second segment has a flat refractive index profile and the third segment has a rounded step type refractive index profile. The Δ1%, Δ2% and Δ3% are respectively 0.73%, 0.012% and 0.18% and the r1, r2 and r3 are respectively 3.4 microns, 9.0 microns and 9.6 microns.