A sensor utilizing attenuated total reflection is equipped with a dielectric block, a thin film layer formed on a surface of the dielectric block, an optical system for making a light beam enter the dielectric block at various angles of incidence so that the condition for total internal reflection is satisfied at an interface between the dielectric block and the thin film layer, a photodetector for detecting the light beam satisfying total internal reflection at the interface, and a differential amplifier array for differentiating a signal output from each of the light-receiving elements of the photodetector, in the juxtaposed direction of the light-receiving elements. Every time a measurement is made, a quantity change .DELTA.I' in the differentiated value I' is found by subtracting an initial value from the differentiated value I'. The quantity change .DELTA.I' is amplified, so that measurements can be made without saturating subsequent electric circuits.