A melting curve analysis is performed for a nucleic acid containing a mutation in a nucleotide sequence resulting in a mutation replacing serine at position 20 in an amino acid sequence of the pancreatic islet amyloid polypeptide with glycine (IAPP S20G), by using a nucleic acid probe of which end is labeled with a fluorescent dye, and in which fluorescence of the fluorescent dye decreases upon hybridization, wherein the nucleic acid probe has a nucleotide sequence complementary to a nucleotide sequence ending at the nucleotide number 247 in the nucleotide sequence of SEQ ID NO: 1 and having a length of 13 to 30 nucleotides, and the 5' end of the probe is labeled with the fluorescent dye, and measuring fluorescence of the fluorescent dye, and the mutation is detected on the basis of the result of the melting curve analysis.