The present invention relates to a novel amino acid dehydrogenase, DNA encoding the enzyme, and a transformant having the DNA introduced therein. The present invention also relates to a process for producing an L-amino acid, 2-oxo acid or D-amino acid, which includes allowing the amino acid dehydrogenase or a microorganism or transformant capable of producing the enzyme to act on a substrate compound. The amino acid dehydrogenase has good reactivity even with an amino acid or a 2-oxo acid each having a bulky side chain such as an aromatic-ring-containing group, which acids are poorly reactive with conventional amino acid dehydrogenases. The amino acid dehydrogenase enables the inexpensive and highly efficient production of a useful optically active amino acid.