A process for efficiently producing optically active amino compounds, mainly (R)-amino compounds, at a low cost by means of a microbial enzyme; a polypeptide suitable for use in the production process and having a stereoselective transaminase activity; and a DNA coding for the polypeptide. The process is characterized by comprising causing a transaminase to act on a ketone compound as an amino acceptor in the presence of a primary amine as an amino donor to stereoselectively transfer the amino group to thus obtain an optically active amino compound. The DNA contains a base sequence coding for a polypeptide having a stereoselective transaminase activity. The polypeptide can be obtained by incubating a microorganism belonging to the genus Arthrobacter.