To develop various functions of a taper optical fiber by increasing optical density by confining light in a comparatively slender tapered part of a taper optical fiber and inducing optical interference in the tapered part.

The taper optical fiber is provided having a tapered part, the part of the taper optical fiber whose diameter varies along the longitudinal direction, and the refractive index is periodically modulated along the propagation direction of light so that the tapered part may induce an optical interference effect. Preferably, the shape of the tapered part has a periodic structure to induce the optical interference effect.