A sensor unit includes a dielectric block, a thin film layer and a reference surface. The thin film layer is formed on the upper surface of the dielectric block and the reference surface is coplanar with the upper surface of the dielectric block. The sensor unit is held in a predetermined position. A light beam is caused to enter the dielectric block to impinge upon the interface between the upper surface of the dielectric block and the thin film layer so that total internal reflection conditions are satisfied at the interface. Information on an analyte on the thin film layer is obtained on the basis of the light beam reflected at the interface. Displacement of the interface is measured by measuring displacement of the reference surface and the position of the sensor unit is adjusted according to the displacement of the reference surface.