The present invention provide a composition for a chemical conversion treatment capable of forming a chemical conversion film having both an excellent black appearance such that the L-value of the film is 28 even when the film is formed from the composition which has been aged, and good corrosion resistance. The composition is a water-soluble composition and comprising a trivalent chromium-containing substance, a cobalt-containing substance, a sulfur compound, and an organic phosphonate compound consisting of one or more compounds selected from the group consisting of organic phosphonic acids, ions of organic phosphonic acids, and organic phosphonates, and a nickel-containing substance as needed. It is preferable that the content of the trivalent chromium-containing substance be 1 to 10 g/L in chromium content equivalent, that the content of the cobalt-containing substance be 0.1 to 10 g/L in cobalt content equivalent, that the content of the sulfur compound be 0.1 to 10 g/L, and that the content of the organic phosphonate compound be 0.1 to 20 g/L. When the composition further contains a nickel-containing substance, the content in nickel content equivalent is preferably 0.10 to 10 g/L.