A surface-treating composition which is a treating liquid containing no ingredients harmful to the environment; such a treating liquid has been difficult to obtain with any conventional technique. The composition enables a coating film having excellent corrosion resistance after coating to be deposited through surface treatment on a surface of a metallic material, e.g., an iron-based metallic material. The composition, which is for the surface treatment of a metal comprising iron and/or zinc, comprises the following ingredients (A), (B), and (C): (A) a compound containing at least one element selected from the group consisting of titanium, zirconium, hafnium, and silicon; (B) a compound containing yttrium and/or a lanthanide element; and (C) nitric acid and/or a nitric acid compound. In the composition, the ratio of the total mass concentration B of the yttrium and/or lanthanide element in the ingredient (B) to the total mass concentration A of the element(s) in the ingredient (A), K1=B/A, is 0.05≦K1≦50 and the ratio of the total mass concentration C of nitrogen atoms in the ingredient (C) in terms of NO3 concentration to the total mass concentration A, K2=C/A, is 0.01≦K2≦200.