A method for isolating a nucleic acid from a biological sample includes applying particulate matter to promote co-aggregation and co-precipitation of insoluble aggregate by directly adding to the biological sample, adding to the biological sample in admixture with a cell lysis buffer, adding to the biological sample treated with a cell lysis buffer, adding to cell lysates in admixture with a buffer for forming denatured protein aggregate; or adding to cell lysates comprising the formed denatured protein aggregate. The particulate matter is selected from the group consisting of a material formed from an element of Ag, Fe, Ti, Al, Sn, Si, Cu, Mo, Ni, W or Zn, an oxide, a carbide, a nitride, a boride and a silicide thereof, and a mixture thereof, a polymer selected from PMMA (Poly Methyl MethAcrylate), polyethylene or polyurethane; and a mixture thereof. The insoluble aggregate comprises denatured protein aggregate and cell debris.