This invention provides a technique enabling to detect target molecules of low concentration with high sensitivity. This invention includes (i) a step of introducing a hydrophilic solvent (42) containing beads (40),(41′) into a space (30) between (a) a lower layer section (10) including a plurality of receptacles (13) each of which is capable of storing only one of the beads (41),(41′) and which are separated from each other by a side wall (12) having a hydrophobic upper surface and (b) an upper layer section (20) facing a surface of the lower layer section (10) on which surface the plurality of receptacles (13) are provided; and (ii) a step of introducing a hydrophobic solvent (43) into the space (30), the step (ii) being carried out after the step (i).