A pretreatment method of extracting nucleic-acid by means of a solid-phase extraction method prior to a nucleic-acid-amplifying process, the pretreatment method comprising: 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, thereby allowing, in advance of the nucleic-acid-amplifying process, to omit both a drying process and an elution process; the predetermined range is defined as follows such that based on 100% of nucleic-acid contained in the sample 5 through 38% of the nucleic-acid is recovered from the sample: the concentration of the silica particles is 0.0625 through 4 [μg/μl]; an average particle diameter of the silica particles is 0.01 through 100 [μm]; and a surface area calculated based on the average particle diameter is 1 x 10 4 through 1 x 10 8 [μm 2]; and the pretreatment method further comprises: a first extraction process of adding the sample to the extraction liquid to extract the nucleic-acid contained in the sample; a first absorption process of: making the silica particles contact with the extracted nucleic-acid to obtain the composite material of the nucleic-acid and the silica particles; and making the composite material contact with the filtering material; and a first washing process of: washing the composite material and the filtering material with purified water; and delivering the washed composite material and the washed filtering material to the nucleic-acid-amplifying process. 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, thereby allowing, in advance of the nucleic-acid-amplifying process, to omit both a drying process and an elution process; the predetermined range is defined as follows such that based on 100% of nucleic-acid contained in the sample 5 through 38% of the nucleic-acid is recovered from the sample: the concentration of the silica particles is 0.0625 through 4 [μg/μl]; an average particle diameter of the silica particles is 0.01 through 100 [μm]; and a surface area calculated based on the average particle diameter is 1 x 10 4 through 1 x 10 8 [μm 2]; and the pretreatment method further comprises: a first extraction process of adding the sample to the extraction liquid to extract the nucleic-acid contained in the sample; a first absorption process of: making the silica particles contact with the extracted nucleic-acid to obtain the composite material of the nucleic-acid and the silica particles; and making the composite material contact with the filtering material; and a first washing process of: washing the composite material and the filtering material with purified water; and delivering the washed composite material and the washed filtering material to the nucleic-acid-amplifying process. the concentration of the silica particles is 0.0625 through 4 [μg/μl]; an average particle diameter of the silica particles is 0.01 through 100 [μm]; and a surface area calculated based on the average particle diameter is 1 x 10 4 through 1 x 10 8 [μm 2]; and the pretreatment method further comprises: a first extraction process of adding the sample to the extraction liquid to extract the nucleic-acid contained in the sample; a first absorption process of: making the silica particles contact with the extracted nucleic-acid to obtain the composite material of the nucleic-acid and the silica particles; and making the composite material contact with the filtering material; and a first washing process of: washing the composite material and the filtering material with purified water; and delivering the washed composite material and the washed filtering material to the nucleic-acid-amplifying process. a first extraction process of adding the sample to the extraction liquid to extract the nucleic-acid contained in the sample; a first absorption process of: making the silica particles contact with the extracted nucleic-acid to obtain the composite material of the nucleic-acid and the silica particles; and making the composite material contact with the filtering material; and a first washing process of: washing the composite material and the filtering material with purified water; and delivering the washed composite material and the washed filtering material to the nucleic-acid-amplifying process.