An object of the invention is to provide an industrially advantageous process capable of remarkably suppressing the generation of an undesirable by-product cepham compound to thereby efficiently produce a desired 2α-methyl-2β-[(1,2,3-triazol-1-yl)methyl]penam-3α-carboxylic acid ester. In the present invention, a diphenylmethyl 2β-bromomethyl-2α-methylpenam-3α-carboxylate (BMPB) is reacted with 1,2,3-triazole in a halogenated hydrocarbon solvent at −5° C. or lower. The reaction in a halogenated hydrocarbon solvent at −5° C. or less can remarkably suppress the generation of an undesirable by-product cepham compound, so that the desired diphenylmethyl 2α-methyl-2β-[(1,2,3-triazol-1-yl)methyl]penam-3α-carboxylate (TMPB) can be efficiently produced.