Disclosed herein is a sensor utilizing attenuated total reflection. The sensor is constructed of a measuring chip, an optical system, and a measuring section. The measuring chip is equipped with a dielectric block, a thin film layer formed on the dielectric block, and a liquid-sample holding portion for holding a liquid sample. The optical system is used to make a light beam enter the dielectric block at various angles of incidence so that a total internal reflection condition is satisfied at an interface between the dielectric block and the thin film layer. The measuring section measures the state of attenuated total reflection, based on the intensity of the light beam totally reflected at the interface. The liquid-sample holding portion has an opening at its top surface. The sensor further has a lid supply mechanism for placing a lid on the opening to prevent evaporation of the liquid sample.