A device for pre-loading a pre-loading spring of a rolling curtain that can be installed between said curtain and a lateral end piece for fixing the curtain to a load-bearing structure, said device can be installed coaxially at at least one end of a screen tube on which a rolling curtain or mosquito screen is rolled and in that it comprises, in combination: - an external support, tubular, on which the screen tube can be fitted, having a toothed inner area with a substantially saw-tooth profile; - a spring pin fitted at the end of the pre-loading spring; - an engagement pin that has the dual function of connection between the device and the lateral end piece for fixing the mosquito screen, as well as between the device and the pre-loading spring through said spring pin; wherein said engagement pin has a distal-end area having a square or polygonal cross section, located on which is the snap-action spring; - an internal bushing that during operation has the function of bearing between the retaining spring and the snap-action spring, as well as the function of longitudinal-guide bearing between the cylindrical body of the engagement pin and the external support, which is the only element that turns during winding-unwinding of the pre-loading spring, i.e., during closing-opening of the mosquito screen; characterized in that the device comprises - a snap-action spring that is substantially constituted by a bushing with one or more saw-tooth external teeth, designed to undergo elastic deformation in a radial direction and co-operate with said internal toothed area of the external support, where the teeth of said snap-action spring are elastically deformable and project in a radial, and not axial, direction with respect to the axis of the rolling curtain; wherein said elements are positioned coaxially to one another and so as to share the axis of the rolling curtain. - an external support, tubular, on which the screen tube can be fitted, having a toothed inner area with a substantially saw-tooth profile; - a spring pin fitted at the end of the pre-loading spring; - an engagement pin that has the dual function of connection between the device and the lateral end piece for fixing the mosquito screen, as well as between the device and the pre-loading spring through said spring pin; wherein said engagement pin has a distal-end area having a square or polygonal cross section, located on which is the snap-action spring; - an internal bushing that during operation has the function of bearing between the retaining spring and the snap-action spring, as well as the function of longitudinal-guide bearing between the cylindrical body of the engagement pin and the external support, which is the only element that turns during winding-unwinding of the pre-loading spring, i.e., during closing-opening of the mosquito screen; characterized in that the device comprises - a snap-action spring that is substantially constituted by a bushing with one or more saw-tooth external teeth, designed to undergo elastic deformation in a radial direction and co-operate with said internal toothed area of the external support, where the teeth of said snap-action spring are elastically deformable and project in a radial, and not axial, direction with respect to the axis of the rolling curtain; wherein said elements are positioned coaxially to one another and so as to share the axis of the rolling curtain.