A surface plasmon resonance sensor comprises a prism having parallel surfaces, a light source provided on one surface of the prism, a detecting means provided on the same surface where the light source is provided, a metal thin film provided on the other surface of the prism, and at least one of light absorbing members provided in the prism for blocking light emitted from the light source at any angle except a predetermined angle so that the metal thin film is illuminated by the light emitted from the light source at the predetermined angle, and such light is totally reflected at the metal thin film so as to reach the detecting means.