Among inorganic clathrates, the inner cavity space rarely affects the electronic structure of the framework. We report that the anti-ReO3-type compound Na3N has a metallic nature irrespective of the stoichiometric chemical composition of simple representative elements and that this unusual nature originates from the collapse of the bandgap owing to the presence of a crystallographic cavity. We synthesized Na3N by the plasma-assisted nitridation of alkali metals, and diffuse reflectance measurements indicated a metallic nature. The introduction of nitrogen into the Na metal induced the formation of both the Na+ ion and the crystallographic cavity. The former increased the density of the lattice of Na+ ions to form a wide Na 3s conduction band. The latter interacted with the Na 3s band to enhance the bandwidth, resulting in the collapse of the bandgap. Na3N is a unique nitride, which possesses an electronically active cavity space.