1. A nucleotide sequence-detecting method, comprising: (a) preparing a nucleotide sample; (b) preparing a first intramolecular detecting sequence having a sequence complementary to a first sequence located at a 3′-side of the detecting site contained in the nucleotide sample and a second intramolecular detecting sequence having a sequence complementary to a second sequence located at a 5′-side of the detecting site, wherein at least one of the 3′-terminal nucleotide of the first intramolecular detecting sequence and the 5′-terminal nucleotide of the second intramolecular detecting sequence is modified in such a manner that they can bind to each other; (c) preparing a detecting chain containing a sequence of the detecting site by connecting the first intramolecular detecting sequence to the 3′ terminal of the nucleotide sample and the second intramolecular detecting sequence to the 5′ terminal; (d) allowing intramolecular hybridization at two positions of the detecting chain between the first sequence and the first intramolecular detecting sequence and between the second sequence and the second intramolecular detecting sequence; (e) connecting the 3′ terminal of the first intramolecular detecting sequence to the 5′ terminal of the second intramolecular detecting sequence directly or indirectly; (f) obtaining a cyclic structure by the connection (e); and (g) detecting the desired sequence in the nucleotide sample from the cyclic structure.