An object of the present invention is to provide a biaxially oriented film suitable as a reinforcing member for an electrolyte membrane of a polymer electrolyte fuel cell, the film having excellent hot water resistance in a high-temperature and high-humidity use environment and being capable of maintaining the reinforcing effect for a prolonged period of time. The invention resides in a biaxially oriented film for reinforcing an electrolyte membrane of a polymer electrolyte fuel cell, the film being characterized in that the film (i) contains syndiotactic polystyrene as a major component and (ii) has a Young's modulus in at least one of the machine direction and the transverse direction ranging from 4,500 to 8,000 MPa.