Disclosed is a measles virus mutant antigen, comprising at least one protein antigen selected from the group consisting of (I) a measles virus mutant H protein antigen and (II) a measles virus mutant F protein antigen, the measles virus mutant H protein antigen (I) being at least one member selected from the group consisting of the whole protein and fragmentary peptides identified with the positional amino acid numbers of either SEQ ID NO: 2 or SEQ ID NO: 10; and the measles virus mutant F protein antigen (II) 4 being at least one member selected from the group consisting of the whole protein and fragmentary peptides identified with the positional amino acid numbers of either SEQ ID NO: 18 or SEQ ID NO: 20. Also disclosed is a measles virus mutant gene coding for the measles virus mutant antigen. By the use of the measles virus mutant antigen or the gene coding for the same of the present invention, it has become possible to provide efficiently and economically a live attenuated measles vaccine or gene vaccine which is adapted for an epidemic strain of measles virus, and a diagnostic reagent capable of accurately detecting infections with an epidemic strain of measles virus.