To provide an environmentally-friendly reacting method in an aqueous medium by using a Lewis acid, capable of promoting reaction in the aqueous medium and further simply separating and reusing.

This reacting method comprises carrying out reaction in an aqueous medium by using a bisperfluoroalkylsulfonylimide metal salt, a trisbisperfluoroalkylsulfonylimide metal salt or a perfluoroalkylsulfonic acid metal salt.