To provide a method for detecting a weak interaction of a biomolecule and a specimen, and to provide a detection device therefor.

The method for detecting the interaction of the biomolecule fixed to a substrate and the specimen labelled with fluorescence using near field light includes a process for bringing the biomolecule and the specimen into contact with each other in a liquid, a process for irradiating the surface of the biomolecule with near field light using an SNOM probe in the liquid and a process for detecting the fluorescence by near field light. The device for detecting the interaction of the biomolecule fixed to the substrate and the specimen labelled with fluorescent using the near field light is equipped with an irradiation means for irradiating the surface of the biomolecule coming into contact with the specimen in the liquid with the field light and a detection means for detecting the fluorescence excited by the field light. Further, a substrate to which at least one biomolecule is fixed can be also used.