The present invention relates to a compound of the formula [I]: wherein R1 is lower alkyl, hydroxy(lower)alkyl or halo(lower)alkyl, and R2 is hydrogen or amino protecting group, or R1 and R2 are bonded together and form lower alkylene or lower alkenylene; R3 is hydrogen or lower alkyl; R4 is R5 is carboxy or protected carboxy; and R6 is amino or protected amino, or a pharmaceutically acceptable salt thereof, a process for preparing a compound of the formula [I], and a pharmaceutical composition comprising a compound of the formula [I] in admixture with a pharmaceutically acceptable carrier.