A process for preparing an allenyl .beta.-lactam compound represented by the formula (4), characterized by reacting the hydroxyl group of a .beta.-lactam compound represented by the formula (1) with a reactive functional group derivative of sulfonic acid represented by the formula (2) to convert the compound of the formula (1) to a .beta.-lactam compound represented by the formula (3), thereafter reacting the resulting .beta.-lactam compound with a basic anion exchange resin of the type having a tertiary organic base fixed to the resin and isolating the resulting allenyl .beta.-lactam compound of the formula (4) from the reaction mixture ##STR1##