To solve a problem occurring in the PALSAR method that a polymer would be formed in the state of unbound to a captured test gene and thus affect the quantitative characteristics as a nonspecific signal, it is intended to develop a technique whereby the polymer formation is controlled in the step of forming an assembly (polymer) of probes so that the polymer is formed exclusively on a test gene to thereby improve the sensitivity and quantitative characteristics. It is found that the polymer can be quantitatively formed and a nonspecific reaction can be inhibited by, in the step of forming a polymer by reacting plural kinds of probes having abilities to complementarily bind to each other, not adding or reacting these probes at once but starting with the reaction of a first probe in one group, and then reacting the second probe in the other group followed by the reactions of probes one by one (i.e., the first probe, the second probe, and so on).