Composition for measuring the binding affinity between nucleic acid and test substance, and use thereof

A method for measuring a binding affinity between a nucleic acid and a test substance, comprising:

• a first measuring step for measuring fluorescence emitted in response to irradiation of light onto a first solution obtained by mixing a nucleic acid with an organic fluorescent substance which is capable of binding to an RNA and which emits fluorescence having an intensity greater while the organic fluorescent substance is liberated from the RNA than while the organic fluorescent substance is bound to the RNA;
• a second measuring step for measuring fluorescence emitted in response to irradiation of light onto a second solution obtained by further mixing the first solution with a test substance;
• and
• a comparing step for comparing (i) the fluorescence measured in the first measuring step with (ii) the fluorescence measured in the second measuring step.