The purpose of the present invention is to provide an optical fiber structure that makes it possible to meet requirements regarding a desired output power, propagation distance, and beam quality. The present invention makes it possible to sufficiently cut off modes equal to or greater than a desired high-order mode and select a region in which Aeff is large in PCF design by choosing a PCF structure having a hole with a diameter d and an interval Λ in a region in which an Aeff region equal to or greater than a desired value and a cutoff region for a desired high-order mode overlap on a graph with a horizontal axis d/ Λ and a vertical axis Λ.
