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 Nas 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.