A sensor comprising a light source for emitting a light beam, and a measuring chip. The measuring chip includes a dielectric block transparent to the light beam, a thin film layer formed on the dielectric block, and a liquid-sample holding mechanism for holding a liquid sample. The sensor also comprises an optical system for making the light beam enter the dielectric block at an angle of incidence so that a total internal reflection condition is satisfied at an interface between the dielectric block and the thin film layer. The sensor further comprises a photodetector for detecting the intensity of the light beam totally reflected at the interface, and a measuring section for measuring a state of attenuated total reflection, based on the result of detection obtained by the photodetector. The irradiation energy of the light beam at the interface is 100 mJ/mm² or less.