Because low molecular weight compounds have simple structures, there is a strong possibility that multiple proteins will bind to them. Competitive inhibition tests are performed in order to identify the target protein from among the multiple proteins recovered. When the same compound that is immobilized on the FG beads® is added to the homogenate in advance, it competes with the target protein to bind to the beads. As a result, when the amount of the compound added is increased, the amount of the recovered target protein decreases. At the same time, because no competitive binding occurs with the non-specific proteins, the recovered amounts of those proteins do not change. When a change in the amount of the recovered target protein occurs depending on whether or not a certain compound is added, then the compound may be the target protein.