The present invention is directed to a process for preparing an exo-methylenepenam compound wherein a cephem compound is reduced with a metal having a standard oxidation-reduction potential of up to −0.3 (V/SCE) in an amount of at least one mole per mole of the cephem compound and with a metal compound having a higher standard oxidation reduction potential than the metal and in an amount of 0.001 to 10 moles per mole of the cephem compound and wherein X is a halogen atom, lower alkylsulfonyloxy, trifluoromethanesulfonyloxy, trichloromethanesulfonyloxy, arylsulfonyloxy, toluenesulfonyloxy, or halogenated sulfonyloxy