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 2.alpha.-methyl-2.beta.-[(1,2,3-triazol-1-yl)methyl]penam-3.alpha- .-carboxylic acid ester. In the present invention, a diphenylmethyl 2.beta.-bromomethyl-2.alpha.-methylpenam-3.alpha.-carboxylate (BMPB) is reacted with 1,2,3-triazole in a halogenated hydrocarbon solvent at -5.degree. C. or lower. The reaction in a halogenated hydrocarbon solvent at -5.degree. C. or less can remarkably suppress the generation of an undesirable by-product cepham compound, so that the desired diphenylmethyl 2.alpha.-methyl-2.beta.-[(1,2,3-triazol-1-yl)methyl]penam-3.alpha.-carbox- ylate (TMPB) can be efficiently produced.