To satisfy a desired transmission characteristic, to prevent generation of an opaque crystal substance of silica or generation of bubbles, and to improve the productivity by forming an outside diameter of a center core part of a segment core type optical fiber, an inside diameter and an outside diameter of a segment core part so as to satisfy the specific relationship, and specifying an inclination of the refractive index distribution of the segment core part.

A single mode optical fiber has an area A on which a specific refractive index difference to quartz is Δn2, an area B on which a specific refractive index difference is Δn0 and an outside diameter formed on the outer periphery of the area A is (b), an area C on which a specific refractive index difference is Δn1 and an outside diameter formed on the outer periphery of the area B is (c) and an area D on which a specific refractive index difference is Δn0 and which is formed on the outer periphery of the area C, and satisfies the relationship of (0.43≤a/b≤0.50 and 0.35≤a/c≤0.40). The refractive index distribution of the area C is formed in an almost isosceles triangle shape having a height of (Δn1-Δn0) and the base length of (c-b)/2.