A high-quality oxide film which is free from a pinhole and surface roughing caused by anodic oxidation and which has surface smoothness on a surface of a material to be treated containing a metal as a principal component. An electrolyte solution which is used for forming an oxide film on a surface of a material to be treated containing a metal as a principal component by anodic oxidation, the electrolyte solution containing a non-aqueous solvent containing an alcoholic hydroxyl group and having 4 or more carbon atoms as a main solvent. This non-aqueous solvent preferably contains two or more alcoholic hydroxyl groups and is especially preferably one or two or more members selected from the group consisting of diethylene glycol, triethylene glycol and polyethylene glycol. A method of forming an oxide film including a step of anodically oxidizing a material to be treated containing a metal as a principal component in this electrolyte solution