A dispersion shifted optical fiber includes: a first core having a constant refractive index within a predetermined radius from the center of an optical fiber, a second core which covers the first core and has a refractive index which decreases from the refractive index of the first core with an increase in its radius, and a cladding which covers the second core and has a refractive index smaller than the minimum refractive index of the second core. Accordingly, the core structure of an optical fiber is controlled, and the optical fiber has a refractive index distribution being a complex of a staircase type having low dispersion and a triangular type having low loss, thus resulting in an optical fiber having low dispersion and low loss.