A compound represented by the general formula (I):
wherein R¹ is a hydrogen atom or a methyl group; R² and R³, which may be the same or different, each represents a hydrogen atom or a lower alkyl group, or taken together with the adjacent nitrogen atom, jointly represent a heterocyclic group selected from the group consisting of an aziridinyl group, an azetidinyl group, a pyrrolidinyl group, a piperidino group, a piperazinyl group, a 4-lower alkylpiperazinyl group, a morpholino group and a thiomorpholino group; and A is a single bond, a lower alkylene group, a lower alkenylene group or a lower alkynylene group; or a pharmaceutically acceptable salt or ester thereof. The compound of the general formula (I) and their salts or esters exhibit excellent antibacterial activity.