Acylating agents comprising: (A) a 1,2-dicarbonyl compound or its hydroxy-reduction derivative; (B) an enzyme; and (C) at least one compound selected from among (c1) metal compounds and (c2) imide compounds such as N-hydroxyphthalimide. As the 1,2-dicarbonyl compound or its hydroxy-reduction derivative (A), use may be made of biacetyl, 2,3-butanediol, etc. As the metal compounds (c1), use may be made of a cobalt compound such as cobalt acetate. An acyl group can be efficiently introduced into a methine carbon atom by treating a compound carrying a methine carbon atom such as an adamantane derivative by the above acylating agent.