A sensor has a sensing surface, to which a specific substance R to be detected can bind. Further, the sensor has a metal portion, at least a portion of which is exposed at the sensing surface, and in which localized plasmons can be excited. The sensor is used in sensing, in which the substance R to be detected is marked with a fluorescent marker Lu that selectively binds to the substance R to be detected and one of two-photon excitation fluorescence and multi-photon excitation fluorescence of the fluorescent marker Lu is detected. Further, the sensing surface is illuminated with measurement light L1 having a wavelength that can excite localized plasmons in the metal portion and that is an absorption wavelength of the fluorescent marker Lu, at which the fluorescent marker Lu emits one of the two-photon excitation fluorescence and the multi-photon excitation fluorescence.