A measuring apparatus is disclosed which includes a measuring unit equipped with a dielectric block and a thin film layer; an incidence system for making a light beam enter the dielectric block so that a condition for total internal reflection is satisfied at an interface between the dielectric block and the thin film layer; and a photodetector for receiving the light beam totally reflected at the interface. The measuring unit is measured a plurality of times, and a change in the state of attenuated total reflection during the plurality of measurements is detected. The sensor further includes a tilt measurement section for measuring the longitudinal tilt of the interface which changes the incidence angles during the plurality of measurements, and a calculating section for obtaining a measured value in which errors due to the longitudinal tilt have been corrected.