To prepare a film having excellent mechanical strengths by selecting an oriented fluororesin film which shows a specified or higher piercing load when it is set up in an unstressed and unloosened state, and a hemispherical needle having a point of a specified diameter is pierced at a specified speed through the center of the film at right angle with respect to the surface of the film.

This film is the one having a thickness of 20-200μm and obtained by uniaxially orienting at a draw ratio of 1.1-10 or biaxially orienting at a draw ratio of 1.2-100 a film containing a fluororesin such as a copolymer comprising ethylene units and tetrafluoroethylene or monochlorotrifluoroethylene units in a 20/80 to 90/10 molar ratio and 0.1-0.5wt.% aggregated silica. This film should have a maximum piercing load of 1.5kgf or above when this film is set up in a circle with a diameter of 100mm in an unstressed and unloosened state, and a hemispherical needle 1mm in diameter and 0.5mm in the diameter of its tip point is pierced through the center of the film at a speed of 50±5mm/ min at right angles to the surface of the film.