The present invention relates to a compound of the formula [I]: wherein R1 is lower alkyl or hydroxy(lower)alkyl, and R2 is hydrogen or amino protecting group, or R1 and R2 are bonded together and form lower alkylene; R is -A-R6 wherein A is bond, —NHCO—(CH2CO)n—, lower alkylene, —NH—CO—CO— or the like, and R6 is wherein R7, R8, R9 and R10 are independently amino, guanidino, amidino or the like; R4 is carboxy or protected carboxy; and R5 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.