Magnetic particles with a metal coat holding target substance captors are made to react with a target substance contained in a specimen in a solution where the magnetic particles are dispersed in a liquid medium. Subsequently, the dispersion of the magnetic particles is applied to a surface having a periodic structure that is adapted to generate plasmon resonance and a change in the plasmon resonance attributable to the concentration of the target substance held on the magnetic particles fixed magnetically to the surface is optically detected to determine the concentration of the target substance in the specimen.