An inner core has a first refractive index. An outer core having a second refractive index lower than the first refractive index is formed on outer circumference of the inner core. A cladding having a refractive index that is higher than the second refractive index and lower than the first refractive index is formed on outer circumference of the outer core. A diameter of the inner core is enlarged, the second refractive index is decreased, and a center core having a refractive index lower than the first refractive index is formed at a center of the inner core, to increase an effective core area while maintaining wavelength dispersion and bending loss.