An industrially excellent novel process for the preparation of addition salts of basic antibiotics with inorganic acids (such as hydrochloric acid), by subjecting a basic antibiotic-oxalic acid addition salt (II) to salt-exchange with an inorganic acid salt (III) of an alkaline earth metal, wherein A is a basic antibiotic; R10 is a protected functional group to be used in organic synthesis; Ak-E is an alkaline earth metal; and B is an inorganic acid; carbapenem derivative-oxalic acid addition salts represented by general formula (II-I) and being useful as intermediates for the preparation of hydrochlorides of antimicrobial agents; and a process for preparing the same, (II-I) wherein A is a three- to seven-membered ring having at least one nitrogen atom; R1 is hydrogen or the like; R?2 and R5¿ are each hydrogen or a protecting group; R3 is a protecting group; R4 is hydrogen or the like; and R6 is hydrogen or the like.