[Objective] An optical member (110) that prevents an excitation light beam from returning to a light source to destabilize light emission by the light source (104) is provided for use in an apparatus that performs measurement utilizing surface plasmon resonance. [Constitution] The shape of a prism (110) 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 (110). The angle of a surface that the excitation light exits the prism (110) 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 (104), and being scattered within the prism are prevented, and therefore the accuracy of measurements can be improved.
