A biaxially stretched polypropylene film comprising a polypropylene resin and satisfying the following: (a) the crystallite size Sa, which is determined using the Scherrer equation from the full width at half maximum of the α-crystal (040) plane reflection peak as measured by wide-angle x-ray diffraction after treatment for 200 hours at 105°C, is not more than 12.9 nm; and (b) the planar orientation coefficient ΔPa, which is calculated from the birefringence values ΔNyz and ΔNxz with respect to the thickness direction as determined by optical birefringence measurement after treatment for 200 hours at 105°C, is at least 0.013 (where, ΔPa = (ΔNyz + ΔNxz)/2).