PURPOSE: To obtain the subject compd. useful as an antimicrobial agent.

CONSTITUTION: A compd. of formula I [R1 is H, a 1 to 4C alkoxy, etc.; R2 is H, a biologically unstable group, a carboxy protecting group; R3, R4 are each OR11, R11 (R11 is a 1 to 6C alkyl, a 2 to 6C alkenyl, etc.); A is H, an amino protecting group, etc.] and its pharmaceutically acceptable salt. For example, (diphenylmethyl)ester of 7-[ (phenoxyacetyl)amino ]8-oxo3(diethoxyphosphinyl)1- azabicyclo[4, 2,0]octa2-ene2carboxylic acid. The compd. of formula I is obtd. by using, for example, a compd. of formula II (A' is an amino protecting group; R2' is a carboxy protecting group) as a starting raw material, converting the material to the 3-bromocarbacephem of formula III by using LiBr in the presence of an org. solvent and base (e.g.: 2,6-lutidine, etc.), converting this compd. to a compd. of formula IV in the presence of a metal salt, Ni(II)Cl2, removing phenoxyacetyl with PCl5/pyridine/isobutyl alcohol, then acylating the compd. with di-tbutyldicarbonate and subjecting the compd. to deblocking.