A sensor utilizing attenuated total reflection (ATR) is provided with a dielectric block, a thin film layer formed on a surface thereof, a light source for emitting a beam, and an optical system for making the beam enter the dielectric block at various angles of incidence so that a condition for total internal reflection is satisfied at an interface between the dielectric block and the thin film layer. The sensor is further provided with a photodetection section for detecting the ATR, a differentiation section for differentiating signals output from the light-receiving elements of the photodetection section, in a direction in which the light-receiving elements are juxtaposed, and an adjustment section for optically expanding the width of a dark line, corresponding to the ATR, of the beam which falls on the photodetection section, so that the width of the dark line becomes greater than a pitch between the light-receiving elements.