An anodic oxide film forming treatment agent for forming an anodic oxide film on a substrate made of aluminum or an aluminum alloy is made of a viscous substance obtained by increasing the viscosity of an electrolytic solution by a nonionic surfactant. A method of forming an anodic oxide film in which the anodic oxide film forming treatment agent is used includes a contacting step of bringing the anodic oxide film forming treatment agent into contact with the substrate, and an energizing step of using the substrate as an anode, and carrying out conduction of electricity between the substrate and a cathode provided in the anodic oxide film forming treatment agent.