It is an object of the present invention to provide a novel amidase that is useful for production of an optically active amino acid, and in particular, a D-amino acid, and a production method thereof.The present invention relates to a novel D-amidase isolated and purified from the Arthrobacter sp. KNK1101J, a gene encoding the above amidase, a recombinant plasmid comprising the above gene, and a transformant into which the above amidase gene has been introduced. In addition, the present invention also relates to a method for producing the amidase, comprising culturing the Arthrobacter sp. KNK1101J or the above transformant, and collecting the above amidase.