A compound of formula (I) in which R1 is hydrogen, optionally substituted lower alkylcarbamoyl(lower)alkylidene, lower alkylidene, lower alkyl, optionally substituted ar(lower)alkyl, cyclo(lower)alkyl(lower)alkyl, protected carboxy(lower)alkyl, carboxy(lower)alkyl, hydroxy(lower)alkyl, optionally substituted lower alkylcarbamoyl(lower)alkyl, lower alkylthio(lower)alkyl, carboxy(lower)alkanoyl, protected carboxy(lower)alkanoyl, aroyl, lower alkanoyl, or optionally substituted arylcarbymoyl(lower)alkyl; R2 is hydrogen, carboxy, protected carboxy, formyl or N-(lower)alkyl-N-(lower)alkoxycarbamoyl; R3 is hydrogen or amidino-protective group; A is lower alkylene or carbonyl; X is (1), (2) or (3); Y is lower alkylene, -S- or -SO¿2?-; Z is -S- or -O-; and the line: ----- is a single bond or a double bond, or pharmaceutically acceptable salts thereof, which is useful as a medicament.