According to an aspect of the present invention, a pair of oligonucleotide strands are anchored onto the surface of a substrate by immobilizing one of the ends thereof onto the substrate. Each of the immobilized oligonucleotide strands is bound to a target nucleic acid sequence (single-stranded) having complementary sequences thereto to form a cross-linked structure on the substrate, thereby forming a finely reticulated space. Ligands are captured by this reticulated space through physical adsorption and caused to color with active substances reactive to the ligands. As a result of this, the present invention is capable of highly sensitively detecting even an exceedingly small concentration of a particular target nucleic acid sequence to be detected, at low cost and for a short time.