There is provided a novel optical sensor utilizing a surface plasmon resonance technique which is capable of detecting a substance to be detected with high sensitivity independently of a wavelength of irradiated light and is capable of obtaining information, other than a refraction index, on the substance to be detected. At the center of a surface of a metallic film 2 which is formed on a substrate and has no aperture, a circular depression 4 with a diameter of 0.1 to 250 nm is formed and with the depression 4 defined as a center, a plurality of depressions 3 are concentrically formed at intervals of 450 to 530 nm.