To obtain an acid catalyst for synthesis of trioxane excellent in heat resistance, less liable to deteriorate and less liable to cause the elimination of the functional group of the acid catalyst by supporting a fluorine-contg. org. material having a sulfone group, a carboxy group or a phosphoric acid group on an inorg. porous body and immobilizing it.

A fluorine-contg. org. material having a functional group for functioning as an acid catalyst for synthesis of trioxane, e.g. a sulfonic acid group, a carboxylic acid group or a phosphoric acid group is supported on an inorg. porous body and the functional group is substantially immobilized to obtain the objective acid catalyst for synthesis of trioxane. The shape of the inorg. porous body is particulate, the average particle diameter is 1 μm to 1 mm, preferably about 20-500 μm, the porosity is 0.20-0.90, preferably about 0.60-0.90 and the average pore diameter is 10-5,000 nm, preferably about 500-2,000 nm. The rate of elution of the supported fluorine-contg. org. material is 0-70%, preferably 0 to about 50%.