An optical fiber includes a core with radius r1, a first clad layer with outermost radius r2 adjacent to the core at radial position r1 and covering the outer periphery of the core, a second clad layer with outermost radius r3 adjacent to the first clad layer at radial position r2 and covering the outer periphery of the first clad layer, and a third clad layer adjacent to the second clad layer at radial position r3 and covering the outer periphery of the second clad layer. The refractive index of the first clad layer decreases continuously from the inside to the outside, reaching a maximum value at radial position r1 and a minimum value at radial position r2. The refractive index of the second clad layer increases continuously from the inside to the outside, reaching a minimum value at radial position r2 and a maximum value at radial position r3.