The present invention provides a method for detecting a target substance, including the steps of: preparing a complex (hybrid) comprising an aptamer which binds to the target substance, a first nucleic acid fragment comprising a nucleotide sequence complementary to the aptamer and a photoisomerizable molecule bound to a portion of the complementary nucleotide sequence, wherein the aptamer and the first nucleic acid fragment bound with the photoisomerizable molecule form double-stranded nucleotides binding; subjecting the photoisomerizable molecule to a first photoisomerization treatment to destabilize the double-stranded nucleotides binding; forming a complex of the target substance and the aptamer so as to dissolve the destabilized double-stranded nucleotides binding; subjecting the photoisomerizable molecule to a second photoisomerization treatment to restabilize double-stranded nucleotides binding; and detecting dissolution of double-stranded nucleotides binding wherein the first nucleic acid fragment bound with the photoisomerizable molecule separates from the aptamer.