The subject of the present invention is to provide a β-lactam acylase protein having high activity, a gene coding for said β-lactam acylase protein, a recombinant vector having said gene, a transformant containing said recombinant vector, and a method of producing a β-lactam antibiotic such as amoxycillin using said β-lactam acylase. A β-lactam acylase gene of Stenotrophomonas maltophilia was cloned, the DNA base sequence and the amino acid sequence expected therefrom was determined, and a Stenotrophomonas β-lactam acylase gene was obtained. This gene was found to code for a protein with a molecular weight of about 70 kDa and having β-lactam acylase activity, and could efficiently produce amoxycillin without being inhibited by phenylacetic acid, etc. Furthermore, by modification of the amino acid sequence, a protein which can more efficiently produce amoxycillin could be obtained.