A method for inexpensively and efficiently producing an optically active amino compound useful as an intermediate for pharmaceutical preparations, agricultural chemicals, or the like, from a ketone compound is provided. Specifically, a polypeptide exhibiting higher activity for glutamic acid as an amino donor than that for L-alanine, and, having novel transaminase activity for generating (S)-1-benzyl-3-pyrrolidinone with high optical purity of 93% or more, a gene encoding the same, and a transformant expressing the gene at a high level are also provided herein.