An impact absorbing unit of a rockfall protection gallery, wherein: it comprises a plurality of columns which are arranged flanked to one another; each column of the plurality of columns comprises: a plurality of collapsible tubular elements; a plurality of connecting elements for connecting the collapsible tubular elements to one another; each collapsible tubular element of the plurality of collapsible tubular elements comprises an end edge; each connecting element of the plurality of connecting elements comprises a first face and a second face, opposite the first face, and is fixed to two collapsible tubular elements (3a, 3b) of a same column of the plurality of columns, which are consecutive to one another; the impact absorbing unit is configured in such a way that, during use, at least a part of the collapsible tubular elements of the plurality of collapsible tubular elements deforms in an irreversible way, by effect of the compression determined by an axial force exerted by a mass falling on the rockfall protection gallery; the impact absorbing unit of a rockfall protection gallery is characterized in that: each collapsible tubular element of the plurality of collapsible tubular elements has a length and a transversal section which are inter-related in order to determine a folding process, i.e. an irreversible compression deformation of the collapsible tubular element which leads to collapse thereof, and therefore a crushing thereof, along the relative extension axis when the collapsible tubular element is subjected to an axial force having an intensity that is at least equal to a critical force value; the end edge of a collapsible tubular element of the two collapsible tubular elements (3a, 3b) is fixed to the relative first face and so that the end edge of the other collapsible tubular element of the two collapsible tubular elements (3a, 3b) is fixed to the relative second face. it comprises a plurality of columns which are arranged flanked to one another; each column of the plurality of columns comprises: a plurality of collapsible tubular elements; a plurality of connecting elements for connecting the collapsible tubular elements to one another; each collapsible tubular element of the plurality of collapsible tubular elements comprises an end edge; each connecting element of the plurality of connecting elements comprises a first face and a second face, opposite the first face, and is fixed to two collapsible tubular elements (3a, 3b) of a same column of the plurality of columns, which are consecutive to one another; the impact absorbing unit is configured in such a way that, during use, at least a part of the collapsible tubular elements of the plurality of collapsible tubular elements deforms in an irreversible way, by effect of the compression determined by an axial force exerted by a mass falling on the rockfall protection gallery; the impact absorbing unit of a rockfall protection gallery is characterized in that: each collapsible tubular element of the plurality of collapsible tubular elements has a length and a transversal section which are inter-related in order to determine a folding process, i.e. an irreversible compression deformation of the collapsible tubular element which leads to collapse thereof, and therefore a crushing thereof, along the relative extension axis when the collapsible tubular element is subjected to an axial force having an intensity that is at least equal to a critical force value; the end edge of a collapsible tubular element of the two collapsible tubular elements (3a, 3b) is fixed to the relative first face and so that the end edge of the other collapsible tubular element of the two collapsible tubular elements (3a, 3b) is fixed to the relative second face. each collapsible tubular element of the plurality of collapsible tubular elements has a length and a transversal section which are inter-related in order to determine a folding process, i.e. an irreversible compression deformation of the collapsible tubular element which leads to collapse thereof, and therefore a crushing thereof, along the relative extension axis when the collapsible tubular element is subjected to an axial force having an intensity that is at least equal to a critical force value; the end edge of a collapsible tubular element of the two collapsible tubular elements (3a, 3b) is fixed to the relative first face and so that the end edge of the other collapsible tubular element of the two collapsible tubular elements (3a, 3b) is fixed to the relative second face.