A process for preparing a penicillanic acid compound of the formula (2) comprising reacting a halogenated penicillanic acid compound of the formula (1), (a) in the presence of a metal bismuth or bismuth compound, (b) with a metal having a lower standard oxidation-reduction potential than bismuth to obtain the compound (2) ##STR00001## (X and Y are a hydrogen atom or halogen atom, provided that X and Y are not hydrogen atoms at the same time, n is an integer of 0 to 2, R is carboxylic acid protecting group) ##STR00002## (n and R are same as above).