To provide a polymer compound capable of forming an alignment film which has a sufficient aligning performance to a liquid crystalline compound and which is excellent also in adhesion with an optical anisotropic film obtainable by polymerizing a liquid crystalline compound in an aligned state, and a composition containing it, as well as an alignment film obtainable by using such a composition, an optical element and an optical information writing/reading device. Provided is a polymer compound comprising polymerized units derived from a fumaric acid diester and polymerized units having a radical polymerizable group in a side chain. By using this polymer compound, an alignment film is prepared. An optical element having this alignment film and an optical anisotropic film is used as a retardation plate 4 having mechanical strength and heat resistance, and good reliability without peeling. An optical information writing/reading device is constituted by using it.