A conveying screw apparatus for transporting loose material. The apparatus includes:—a power unit apt to rotate an Archimedes screw pushing the loose material;—a charging hopper of the loose material in a tubular body containing the Archimedes screw and the loose material; —a feeding channel of the loose material towards a user; —pushing elements of the loose material present in the feeding channel; and —a back flow preventing device, preventing the back flow of pushing air together with loose material; such a device closing, in case, a through hole to the feeding channel. The back flow preventing device includes a shutter, provided with a plate, undergoing the action of a helical spring which, in any opening/closing step of the through hole, is always substantially perpendicular to the feeding first direction of the material in the tubular body.