PURPOSE: To produce the subject fiber on which a beam size expanded region is previously formed at regular intervals over its entire length by heating the part of the fiber sufficiently shorter than the interval to a high temp.

CONSTITUTION: An optical fiber preform is drawn to form a high-strength single- mode optical fiber (2-3) with a thin-film carbon formed around the clad over its entire length. The specified length of the fiber sufficiently smaller than a specified interval L is heated at a thousand several hundred degrees (°C) for a specified time, and an expanded beam region forming single-mode fiber on which a beam size expanded region (2-4) having the beam size larger than that of the single-mode of its own is partially and periodically formed is produced.