PURPOSE: To perform a reaction process and a raw material and reaction product separating process in one stage by using a porous membrane having an ion exchange functional group on the surface thereof and the surfaces of the pores thereof as a reaction catalyst and introducing a raw material solution into one surface of the membrane while introducing an extractant into the other surface thereof.

CONSTITUTION: In preparing a reaction product B (e.g., trioxane) from a raw material solution A (e.g., formalin aqueous solution), a porous membrane 1 having a cation exchange functional group on the surface thereof and the surfaces of the pores thereof is used as a reaction catalyst. Then, the raw material solution A is introduced in one surface of the membrane 1 and an extractant C (e.g., alkylbenzene) not substantially dissolving the raw material solution A but capable of dissolving the reaction product B is introduced into the other surface of the porous membrane 1 and the raw material solution A is transmitted through the porous membrane 1. By this method, a mixture consisting of the solution having the reaction product B and the extractant C dissolved therein and the residual liquid not dissolved in the solution is obtained. As a result, the reaction process and the raw material and reaction product separating process carried out according to two complicated steps heretofore are carried out in one stage.