A method of manufacturing an optical device includes: a first step of forming an optical-device forming body that includes a plurality of columnar structures arranged in an arrangement direction on a substrate surface via a trench and an outline structure connected to and containing therein the plurality of columnar structures; a second step of oxidizing the optical-device forming body from a state where the optical-device forming body starts to be oxidized to a state where the columnar structure is oxidized; and a third step in which an unoxidized residual part of the outline structure in the second step is oxidized after the second step so as to form an oxidized body. Furthermore, the third step includes restraining the outline structure from being deformed with respect to at least the arrangement direction of the columnar structures in the third step.