Disclosed is a low-attenuation single-mode optical fiber having a photoconductive core and a clad, which shows very low OH concentration in MFD (Mode Field Diameter) region. The optical fiber includes a core positioned at its center for photoconduction, and a dehydrated clad and a base clad which are coated on the clad in order. The dehydrated clad has a substantially identical refractive index to the base clad. A refractive index of the core is greater than those of the dehydrated clad and the base clad. The dehydrated clad has a relatively lower OH concentration than the base clad. The region composed of the core and the dehydrated clad has an MFD region at which OH concentration is less than 0.8 ppb.