Provided is a highly insulating film having excellent electrical characteristics (breakdown voltage), heat resistance, and handling properties such as windability and processability. The highly insulating film is a biaxially stretched film made of a styrene polymer having a syndiotactic structure as a main component. The highly insulating film contains specific inert fine particles A, antioxidant, and polymer Y having a glass transition temperature Tg by DSC of 130° C. or more each in a specific amount, and has a refractive index in the thickness direction of 1.5750 or more and 1.6350 or less.