The shape of a prism is set such that an excitation light beam that enters the prism to cause surface plasmon resonance to be generated is not irradiated onto the corners of the prism after being totally reflected within the prism. The angle of a surface that the excitation light exits the prism is set to an angle at which the excitation light beam is not totally reflected. Thereby, the excitation light beam returning to a light source, and being scattered within the prism are prevented, and therefore the accuracy of measurements can be improved.