There is provided a single mode optical fiber including a core layer, a first cladding layer, and a second cladding layer, in which when a refractive index of a center portion of the core layer is set as no, a radial position where a refractive index becomes n.sub.00.45 is set as r.sub.CORE, a minimum value of a refractive index of the first cladding layer is set as n.sub.3, and a radial position where a refractive index becomes the n.sub.3 is set as r.sub.3, a relative refractive index difference of the first cladding layer is reduced continuously and gently from the r.sub.CORE to the r.sub.3.