A plasmon sensor apparatus using a metallic fine periodic structure designed to reduce the dependences of the resonance wavelength and sensitivity on the incident angle. The plasmon sensor apparatus has a sensing element including a metallic member having periodic slit openings and metallic portions, and a substrate on which the metallic member is held, a light source which emits light so that the light is incident on the sensing element, and a photodetector which detects light obtained from the light source. If the period of the slit openings is .LAMBDA.; the width of the metallic portions is d; and the thickness of the metallic member is h, the aspect ratio h/(.LAMBDA.-d) of the slit openings is 3 or higher and the opening width (.LAMBDA.-d) is equal to or smaller than the wavelength of light applied from the light source to the sensing element.