An optical signal detection method comprising the steps of: preparing a sensor chip (10, 10', 50, 50' and 61) including a dielectric plate (11, 51) and a sensor portion (14, 58) having a metal layer (12, 12' and 58a) deposited on a predetermined area of a surface of the dielectric plate (11, 51); binding a binding substance of an amount corresponding to the amount of a substance to be detected that is included in a sample to the sensor portion (14, 58) by contacting the sample with the sensor portion (14, 58) of the sensor chip (10, 10', 50, 50' and 61), the binding substance having a photo-reactable labeling-substance attached to the binding substance; and obtaining the amount of the substance to be detected by irradiating the predetermined area with excitation light (L 0) and by detecting light output from the photo-reactable labeling substance in an enhanced electric field that has been generated on the metal layer (12, 12' and 58a) by irradiation with the excitation light (L 0), characterized in that the photo-reactable labelling substance comprises a light transmissive material and a plurality of molecules of a photo-reactable substance enclosed within said light transmissive material, wherein said light transmissive material transmits light output from the the plurality of molecules of the photo-reactable substance and wherein said photo-reactable labeling substance is of a particle size within a predetermined range that maintains a desired distance between the photo-reactable substance and the metal layer (12, 13 and 58a) to prevent metal quenching that occurs when the photo-reactable substance is located close to the metal layer (12, 13 and 58a). preparing a sensor chip (10, 10', 50, 50' and 61) including a dielectric plate (11, 51) and a sensor portion (14, 58) having a metal layer (12, 12' and 58a) deposited on a predetermined area of a surface of the dielectric plate (11, 51); binding a binding substance of an amount corresponding to the amount of a substance to be detected that is included in a sample to the sensor portion (14, 58) by contacting the sample with the sensor portion (14, 58) of the sensor chip (10, 10', 50, 50' and 61), the binding substance having a photo-reactable labeling-substance attached to the binding substance; and obtaining the amount of the substance to be detected by irradiating the predetermined area with excitation light (L 0) and by detecting light output from the photo-reactable labeling substance in an enhanced electric field that has been generated on the metal layer (12, 12' and 58a) by irradiation with the excitation light (L 0), characterized in that the photo-reactable labelling substance comprises a light transmissive material and a plurality of molecules of a photo-reactable substance enclosed within said light transmissive material, wherein said light transmissive material transmits light output from the the plurality of molecules of the photo-reactable substance and wherein said photo-reactable labeling substance is of a particle size within a predetermined range that maintains a desired distance between the photo-reactable substance and the metal layer (12, 13 and 58a) to prevent metal quenching that occurs when the photo-reactable substance is located close to the metal layer (12, 13 and 58a).