Disclosed is an optical fiber (100) comprising a center core (110) which forms a passageway for transmitting optical signals and has a refractive index N1, and a cladding (140) which encloses the center core (110) and has a refractive index N0. The optical fiber (100) further comprises an upper core (120), which has a distribution of refractive indices increased starting from a refractive index N2 (>N0) at its outer cicumference to the refractive index N1 at its internal circumference, and a minutely depressed refractive index region (130), which is interposed between said upper core (120) and cladding (140) and has a refractive index N3. The refractive index N3 is lower than the refractive index N0. <IMAGE>
