A probe for detecting a mutation(s) in exon 12 of the NPM1 gene, comprising at least one fluorescently labeled oligonucleotide selected from the group consisting of P5, P6, P7, and P1 below: an oligonucleotide comprising a nucleotide sequence complementary to a nucleotide sequence of 12 to 50 consecutive nucleotides containing nucleotides 145 to 156 in SEQ ID NO:2, wherein the nucleotide corresponding to the nucleotide at position 145 is a cytosine labeled with a fluorescent dye, and wherein said nucleotide at position 145 labeled with a fluorescent dye is at the first, second or third position from the 3' end of the oligonucleotide; an oligonucleotide comprising a nucleotide sequence complementary to a nucleotide sequence of 12 to 50 consecutive nucleotides containing nucleotides 145 to 156 in SEQ ID NO:2, except that the nucleotide corresponding to the nucleotide at position 153 is a guanine, wherein the nucleotide corresponding to the nucleotide at position 145 is a cytosine labeled with a fluorescent dye, and wherein said nucleotide at position 145 labeled with a fluorescent dye is at the first, second or third position from the 3' end of the oligonucleotide; an oligonucleotide comprising a nucleotide sequence complementary to a nucleotide sequence of 12 to 50 consecutive nucleotides containing nucleotides 145 to 156 in SEQ ID NO:2, except that the nucleotide corresponding to the nucleotide at position 153 is a guanine and the nucleotide corresponding to the nucleotide at position 154 is a thymine, wherein the nucleotide corresponding to the nucleotide at position 145 is a cytosine labeled with a fluorescent dye, and wherein said nucleotide at position 145 labeled with a fluorescent dye is at the first, second or third position from the 3' end of the oligonucleotide; and an oligonucleotide comprising a nucleotide sequence complementary to a nucleotide sequence of 16 to 50 consecutive nucleotides containing nucleotides 135 to 150 in SEQ ID NO:1, wherein the nucleotide corresponding to the nucleotide at position 135 is a cytosine labeled with a fluorescent dye, and wherein said nucleotide at position 135 labeled with a fluorescent dye is at the first, second or third position from the 3' end of the oligonucleotide. an oligonucleotide comprising a nucleotide sequence complementary to a nucleotide sequence of 12 to 50 consecutive nucleotides containing nucleotides 145 to 156 in SEQ ID NO:2, wherein the nucleotide corresponding to the nucleotide at position 145 is a cytosine labeled with a fluorescent dye, and wherein said nucleotide at position 145 labeled with a fluorescent dye is at the first, second or third position from the 3' end of the oligonucleotide; an oligonucleotide comprising a nucleotide sequence complementary to a nucleotide sequence of 12 to 50 consecutive nucleotides containing nucleotides 145 to 156 in SEQ ID NO:2, except that the nucleotide corresponding to the nucleotide at position 153 is a guanine, wherein the nucleotide corresponding to the nucleotide at position 145 is a cytosine labeled with a fluorescent dye, and wherein said nucleotide at position 145 labeled with a fluorescent dye is at the first, second or third position from the 3' end of the oligonucleotide; an oligonucleotide comprising a nucleotide sequence complementary to a nucleotide sequence of 12 to 50 consecutive nucleotides containing nucleotides 145 to 156 in SEQ ID NO:2, except that the nucleotide corresponding to the nucleotide at position 153 is a guanine and the nucleotide corresponding to the nucleotide at position 154 is a thymine, wherein the nucleotide corresponding to the nucleotide at position 145 is a cytosine labeled with a fluorescent dye, and wherein said nucleotide at position 145 labeled with a fluorescent dye is at the first, second or third position from the 3' end of the oligonucleotide; and an oligonucleotide comprising a nucleotide sequence complementary to a nucleotide sequence of 16 to 50 consecutive nucleotides containing nucleotides 135 to 150 in SEQ ID NO:1, wherein the nucleotide corresponding to the nucleotide at position 135 is a cytosine labeled with a fluorescent dye, and wherein said nucleotide at position 135 labeled with a fluorescent dye is at the first, second or third position from the 3' end of the oligonucleotide.