Abstract: To provide a non-chromate surface treatment agent that, without using one or two or more iron group compounds selected from oxides, hydroxides, or oxyhydroxides of iron, nickel, or cobalt, or the like, allows a surface treatment coating having excellent adhesion to an aluminum material or the like and an upper layer film and providing excellent corrosion resistance to the aluminum material or the like to be formed between the aluminum material or the like and the upper layer film, a method for forming a surface treatment coating using the surface treatment agent, and an aluminum material or an aluminum alloy material having thereon the surface treatment coating. The above challenge can be solved by a surface treatment agent that contains a water-soluble or water-dispersible organic polymer (A) containing a carbonyl group and/or hydroxyl group in a unit structure and an organic polymer (B) containing a phosphonic group.