The present invention provides a birefringent film having an index ellipsoid satisfying a relationship of nz>nx>ny, and having thinness and light weight, which can be relatively easily produced. The birefringent film of the present invention contains an acenaphtho[1,2-b]quinoxaline derivative represented by the following general formula (I), and an index ellipsoid of the film satisfies a relationship of nz>nx>ny. Here, in the formula (I), X, Y, and Z each independently represent a halogen atom, an alkoxy group, an --R group, an --OM group, a --COOM group, an --OCOR group, an --NHCOR group, a --CONHR group, an --NH.sub.2 group, an --NO.sub.2 group, a --CF.sub.3 group, a --CN group, an --OCN group, an --SCN group, an --SM group, and a --PO.sub.3M group; M represents a counterion; R represents hydrocarbon; k, l, m, and n represent the number of substitutions, k is an integer of 1 to 4, and l, m and n are each independently an integer of 0 to 3. ##STR00001##