A cartridge retaining mechanism equipped in a nucleic acid extracting apparatus for extracting a nucleic acid, the mechanism comprising: a cartridge that comprises: a cartridge main body that has a cylindrical shape with a bottom, and the bottom is shaped in a funnel shape; a nucleic acid-adsorbing solid carrier that traps a nucleic acid, and the carrier is disposed at the bottom; and a cartridge cap that is detachably mounted on an open end of the cartridge main body, a supporting part that supports the cartridge; and a pressure-proof retaining part that pushes the cartridge cap, wherein a cylindrical rib with a diameter smaller than a diameter of the cartridge main body is formed on an upper plane of the cartridge cap so that the cylindrical rib protrudes from the upper plane; a nozzle-receiving opening to which a pressure nozzle is pressed is formed at the cylindrical rib; and an aperture through which the cylindrical rib is inserted is formed in the pressure-proof retaining part, and wherein when the pressure nozzle is pressed to the nozzle-receiving opening, along with the nozzle-receiving opening being exposed from the aperture, the cartridge cap is pressed by the pressure-proof retaining part and simultaneously the cartridge main body is supported by the supporting part.