A surface plasmon resonance assay apparatus is loaded with a sensor unit. A sensing surface of a thin film detects reaction of a sample. A dielectric prism is overlaid with the thin film to constitute an interface. A reflection angle upon occurrence of attenuated total reflection of the illuminating light is changeable according to reaction of the sample on the sensing surface. Protecting panels are disposed to face outer surfaces of the prism, for covering and protecting at least partially the outer surfaces. A first window in one of the protecting panels is positioned on a path of the illuminating light traveling for incidence on the interface, for passing the illuminating light. A second window in one remaining protecting panel is positioned on a path of the illuminating light traveling upon reflection by the interface, for passing the illuminating light.