The present invention provides a method for making a harmful arsenic compound, antimony compound and selenium compound harmless by using an organic cobalt complex, in which the cost of the method can be improved. The present invention is a method for making a harmful compound harmless, including irradiating light to an organic cobalt complex containing cobalt as a central metal and a corrin ring as a ligand, a methyl group donor, a titanium oxide photocatalyst, and a harmful compound containing an arsenic atom, an antimony atom or a selenium atom to methylate the harmful compound. In the present invention, it is preferable that the harmful compound be trimethylated.