PURPOSE: To enhance the surface slidability and withstand voltage characteristic by selecting the specific value for a surface projection of a bi-axially oriented polyphenylene sulfide film so as to conform to the specific valve.

CONSTITUTION: In a bi-axially oriented polyphenylene sulfide film including over 70mol% of iterative unit as shown by Eq. I over 70mol% and also incl. 0.1-5mol% of tri-functional unit, which provides possibility of branching of molecular chain, the values concerned shall be selected so that the mean gradient of the distribution curve of the relationship between the film projection height μm and log (number of projections/mm2) will be less than -2 in the range of projection height between 0.1 and 0.4μm, that the number of projections over the height of 0.1μm be 100/mm2, and that the number of projections over 0.4μm be under 1000/mm2. This accomplishes the optimum surface condition in which there are a large number of projections with specific height and less number of greater projections, to yield a film for capacitor with enhanced surface slidability, which is required at the time of processing, and withstand voltage characteristic.