The present invention provides a method capable of realizing pretreatment process of genetic screening according to a POCT mode. The method includes: making a sample, extraction liquid for extracting nucleic-acid contained in the sample, silica particles, and a filtering material contact with each other; making the filtering material carry composite material of the nucleic-acid and the silica particles thereon; and then delivering the filtering material to a nucleic-acid-amplifying process by means of reaction solution for amplifying nucleic-acid, wherein particle diameters of the silica particles and concentration of the silica particles in the reaction solution for amplifying nucleic-acid are set up within a predetermined range.