Process for the preparation of a polyimide film.
A polyimide film essentially consisting of polyimide having recurring units of the formula ( I ): <CHEM> which has a density of 1.335 to 1.390 g/cm<3> at 23 DEG C and/or a refractive index of 1.605 to 1.680 at 23 DEG C in the direction of thickness and is transparent; and a preparation process of the polyimide film by extruding the polyimide having recurring units of the above formula ( I ) throught a common melt-extrusion process, casting an a chill-roller to obtain an unstratched film, uniaxially or biaxially stretching the unstretched film to cause molecular orientation, and successively setting the stretched film through heat-treatment; is disclosed.