The optical fiber coupler of the present invention is characterized in that, for a defined section in which two or more component optical fibers are thermally fused and elongated thereby forming a fused-elongated region which constitutes the optical fiber coupler, that at least one of the component optical fibers is formed from a single mode optical fiber material employed within a parameter range in which the mode field diameter increases monotonically with decrease in the diameter of the optical fiber core. The optical fiber coupler is further characterized in that, when using optical fibers from which a portion of the coating material has been removed thereby exposing the cladding, and by aligning two or more of such component optical fibers side by side in a plane and mutually thermally fusing the exposed cladding of adjacent optical fibers, after which the fused region is drawn out, thereby forming a fused-elongated region which constitutes the optical fiber coupler, for such an optical fiber coupler, optical fiber material is used in which the softening temperature of the core is higher than that of the cladding, and further, for the drawing out of the mutually fused sections, the tension employed is such that by virtue of remaining stress, the refractive index of the core is reduced.