A new metallo-²-lactamase inhibitor which acts as a medicament for inhibiting the inactivation of ²-lactam antibiotics and recovering anti-bacterial activities is disclosed. The maleic acid derivatives having the general formula (I) have metallo-²-lactamase inhibiting activities. It is possible to recover the anti-bacterial activities of ²-lactam antibiotics against metallo-²-lactamase producing bacteria by combining the compound of the general formula (I) with ²-lactam antibiotics.