The present invention provides a means and method useful for measurement of a total branched-chain amino acid concentration. Specifically, the present invention provides a modified enzyme in which at least one amino acid residue is mutated so as to improve a property of a leucine dehydrogenase which is associated with the measurement of the total branched-chain amino acids, such as, for example, substrate specificities of leucine dehydrogenase for total branched-chain amino acids, activity of leucine dehydrogenase for any branched-chain amino acids, and thermal stability of leucine dehydrogenase; and a method of analyzing the total branched-chain amino acids, comprising measuring the total branched-chain amino acids contained in a test sample using the modified enzyme.