of the present invention can measure whether or not cytosine in genomic dna has been methylated this example shows that it is suitable for the biotinylated nucleic acid used in the present invention to be made in a base sequence not forming a base pair with the cytosine to be detected dna was prepared as follows the nucleic acid to be measured used was 5 u2032 ttg cgc ggc gtc cgt cct gtt gac ttc 3 c at the 13th base from the 5 u2032 end is methylcytosine seq id no 3 the nucleic acid to be measured was hybridized to each of the following 6 biotinylated nucleic acids by the same procedure as in example 1 5 u2032 gaa gtc aac agg ac u2014 gac gcc gcg caa 3 u2032 seq id no 4 is designed so that the cytosine to be detected is placed in the bulge 5 u2032 gaa gtc aac agg aca gac gcc gcg caa 3 u2032 seq id no 5 has a mismatch in which the base to form a base pair with the cytosine to be detected is a 5 u2032 gaa gtc aac agg act gac gcc gcg caa 3 u2032 seq id no 6 has a mismatch in which the base to form a base pair with the cytosine to be detected is t 5 u2032 gaa gtc aac ago acc gac gcc gcg caa 3 u2032 seq id no 7 has a mismatch in which the base to form a base pair with the cytosine to be detected is c 5 u2032 gaa gtc aac agg acd gac gcc gcg caa 3 u2032 seq id no 8 has a mismatch in which the base to form a base pair with the cytosine to be detected is.