PROBLEM TO BE SOLVED: To provide an acid catalyst which can reduce the quantity of waste in an organic synthesis reaction and which does not corrode a reactor and is nontoxic.SOLUTION: A phenolic compound having a bis(trifluoromethanesulfonyl)ethyl group is used. The compound can be utilized as a catalyst in various organic chemical reactions, particularly in organic synthesis reactions such as Diels-Alder reaction, Friedel-Crafts reaction, Michael addition reaction and esterification reaction. This acid catalyst exhibits higher activity compared with the conventional Lewis acid, and accordingly need not be used in a stoichiometric amount. Namely, the acid catalyst enables, even when an extremely small amount of the catalyst is used, a desired organic reaction to proceed. Further, the acid catalyst is soluble in various solvents and thus enables homogeneous reaction, and further the acid catalyst can be easily and simply separated from an objective product after the completion of reaction. A reaction can be carried out at a relatively low temperature near ordinary temperature. A method of using the compound is highly useful also as an industrial manufacturing process.