A measuring apparatus for measuring the state of attenuated total reflection over time for a single measuring unit without being affected by the change in measuring conditions arising from replacement and resetting of the sample. A light beam is entered into the interface between a dielectric block and a metal film having a dielectric film thereon at various incident angles within the angle range that creates two or more dark lines due to attenuated total reflections, and the variation in the positions of other dark lines are measured with reference to the dark line having the least positional variation among them.