A method of manufacturing an optical element having a good reflection preventing characteristic is provided in which an irregularity structure having a reflection preventing function can be formed at low temperature. The method includes forming a film on the optical substrate by using a coating liquid containing a metal alkoxide containing at least aluminum, immersing the film in a water-containing liquid, baking the film having been immersed in the water-containing liquid, and subjecting the film having been baked to treatment with warm water to form an aluminum-containing film having a fine irregularity structure.