A measuring apparatus includes a sensor well unit having a plurality of sample wells, which are formed in a dielectric block and a thin film layer provided on the inner bottom surface of each sample well. A light beam projector causes a plurality of light beams to impinge upon the interfaces of the inner bottom surfaces of one-dimensionally arranged sample wells out of the plurality of sample wells and the thin film layers at various angles of incidence so that total internal reflection conditions are satisfied at each of the interfaces, and the light beams reflected at the respective interfaces are received by a photodetector. A longitudinal tilt measuring system measures a longitudinal tilt of the interface from a predetermined reference position, and a corrected measured value corrected according to the longitudinal tilt measured by the longitudinal tilt measuring system is obtained.