A sensor utilizing an evanescent wave and comprising a light source for emitting a light beam and a measuring chip formed into the shape of a well. The measuring chip has a dielectric block and a thin film layer formed on one surface of the dielectric block. The sensor also includes an optical system for making the light beam enter the dielectric block at angles of incidence so that a total internal reflection condition is satisfied at an interface between the dielectric block and the thin film layer. The sensor further includes a sample supply-discharge unit which is detachably installed within the measuring chip. The sample supply-discharge unit is used for continuously supplying the sample onto the surface of the thin film layer and continuously discharging the supplied sample from the surface of the thin film layer.