Abstract: The present invention provides a 2-arylmethylazetidine-carbapenem-3-carboxylic acid ester derivative or its pharmaceutically acceptable salt, a process for the preparation thereof, and a pharmaceutical composition comprising the same. The 2-arylmethylazetidine-carbapenem-3-carboxylic acid ester derivatives or their pharmaceutically acceptable salts show high oral absorption rate, and thus can be orally administered. The active metabolites thereof have a broad spectrum of antibacterial activities against Gram-positive and Gram-negative bacteria and excellent antibacterial activities against methicillin-resistant Staphylococcus aurus (MRSA) and quinolone-resistant strains (QRS). In particular, the acid addition salts of the 2-arylmethylazetidine-carbapenem-3-carboxylic acid ester derivatives are obtained in crystalline forms having excellent stability.