In a chemical sensing apparatus utilizing a surface plasmon resonance in a small aperture formed in a metal thin film or on a surface of a metal fine particle, a capturing substance is disposed in the small aperture or on the surface of the metal fine particle for capturing a target substance. A marker substance, having a size comparable to that of the small aperture or the metal fine particle is combined with the target substance. As a result, a spectral change is increased in the transmitted light or the scattered light, induced by a surface plasmon resonance and resulting from the capture of the target substance.