The present invention provides a thin and light-weight birefringent film having a desired Nz coefficient, such that an index ellipsoid satisfies a relationship of nx.gtoreq.nz>ny. The birefringent film of the present invention contains a first acenaphtho[1,2-b]quinoxaline derivative exhibiting lyotropic liquid crystallinity and a second acenaphtho[1,2-b]quinoxaline derivative exhibiting lyotropic liquid crystallinity, wherein an index ellipsoid satisfies a relationship of nx.gtoreq.nz>ny. An Nz coefficient of this birefringent film is preferably 0 to 0.5.