The purpose of the present invention is to provide a high-insulating film having excellent heat resistance and an improved breakdown voltage. This high-insulating film: comprises a biaxially stretched film, the main constituent of which is a styrene-based polymer with a syndiotactic structure; contains 5-48 mass% of a thermoplastic amorphous resin (Y) with a glass transition temperature (Tg) of 130°C or more by DSC; and is characterized by having a plane orientation coefficient (?P), represented by formula (1), of -0.027 or less. ?P = (Nx + Ny)/2 - Nz (1) (In formula (1): Nx represents the minimal value of the refraction index in the surface direction of the film; Ny represents the refraction index in the surface direction of the film and in the direction perpendicular to Nx; and Nz represents the refraction index in the thickness direction of the film.)
