Use of a probe to detect an EGFR exon 21 L858R polymorphism, the probe comprising at least one type of fluorescence-labeled oligonucleotide selected from P1, P3 and P15 - P18 below: An oligonucleotide comprising a complementary sequence to a nucleotide sequence having a length of 11 - 50 bases comprising base positions 251 - 261 of the base sequence of SEQ ID NO: 1 or a sequence having not less than 80% identity with said complementary sequence, and in which the base corresponding to base number 261 is adenine or cytosine, the base corresponding to base number 251 is cytosine, and the cytosine at base number 251 is fluorescence-labeled; An oligonucleotide comprising a complementary sequence to a nucleotide sequence having a length of 5 - 50 bases comprising base positions 257 - 261 of the base sequence of SEQ ID NO: 1 or a sequence having not less than 80% identity with said complementary sequence, and in which the base corresponding to base number 261 is adenine or cytosine, the base corresponding to base number 257 is cytosine, and the cytosine at base number 257 is fluorescence-labeled; An oligonucleotide comprising a complementary sequence to a nucleotide sequence having a length of 6 - 50 bases comprising base positions 259 - 264 of the base sequence of SEQ ID NO: 1 or a sequence having not less than 80% identity with said complementary sequence, and in which the base corresponding to base number 261 is adenine or cytosine, and cytosine positioned at the 3' side relative to the base is fluorescence-labeled; An oligonucleotide comprising a complementary sequence to a nucleotide sequence having a length of 5 - 50 bases comprising base positions 258 - 262 of the base sequence of SEQ ID NO: 1 or a sequence having not less than 80% identity with said complementary sequence, and in which the base corresponding to base number 261 is adenine or cytosine, and cytosine positioned at the 5' side relative to the base is fluorescence-labeled; An oligonucleotide comprising a sequence having not less than 80% identity with a nucleotide sequence having a length of 16 - 50 bases comprising base positions 249 - 264 of the base sequence of SEQ ID NO: 1, and in which the base that is homologous to base number 261 is thymine or guanine, and cytosine positioned at the 5' side relative to the base is fluorescence-labeled; and An oligonucleotide comprising a sequence having not less than 80% identity with a nucleotide sequence having a length of 8 - 50 bases comprising base positions 257 - 264 of the base sequence of SEQ ID NO: 1, and in which the base that is homologous to base number 261 is thymine or guanine, and cytosine positioned at the 3' side relative to the base is fluorescence-labeled. An oligonucleotide comprising a complementary sequence to a nucleotide sequence having a length of 11 - 50 bases comprising base positions 251 - 261 of the base sequence of SEQ ID NO: 1 or a sequence having not less than 80% identity with said complementary sequence, and in which the base corresponding to base number 261 is adenine or cytosine, the base corresponding to base number 251 is cytosine, and the cytosine at base number 251 is fluorescence-labeled; An oligonucleotide comprising a complementary sequence to a nucleotide sequence having a length of 5 - 50 bases comprising base positions 257 - 261 of the base sequence of SEQ ID NO: 1 or a sequence having not less than 80% identity with said complementary sequence, and in which the base corresponding to base number 261 is adenine or cytosine, the base corresponding to base number 257 is cytosine, and the cytosine at base number 257 is fluorescence-labeled; An oligonucleotide comprising a complementary sequence to a nucleotide sequence having a length of 6 - 50 bases comprising base positions 259 - 264 of the base sequence of SEQ ID NO: 1 or a sequence having not less than 80% identity with said complementary sequence, and in which the base corresponding to base number 261 is adenine or cytosine, and cytosine positioned at the 3' side relative to the base is fluorescence-labeled; An oligonucleotide comprising a complementary sequence to a nucleotide sequence having a length of 5 - 50 bases comprising base positions 258 - 262 of the base sequence of SEQ ID NO: 1 or a sequence having not less than 80% identity with said complementary sequence, and in which the base corresponding to base number 261 is adenine or cytosine, and cytosine positioned at the 5' side relative to the base is fluorescence-labeled; An oligonucleotide comprising a sequence having not less than 80% identity with a nucleotide sequence having a length of 16 - 50 bases comprising base positions 249 - 264 of the base sequence of SEQ ID NO: 1, and in which the base that is homologous to base number 261 is thymine or guanine, and cytosine positioned at the 5' side relative to the base is fluorescence-labeled; and An oligonucleotide comprising a sequence having not less than 80% identity with a nucleotide sequence having a length of 8 - 50 bases comprising base positions 257 - 264 of the base sequence of SEQ ID NO: 1, and in which the base that is homologous to base number 261 is thymine or guanine, and cytosine positioned at the 3' side relative to the base is fluorescence-labeled.