Method of making a porous film

A method for manufacturing a porous film having non-through pores by a porous film manufacturing apparatus which comprises:

• a perforating unit including a first roll having a surface on which particles having sharp pointed portions and a Mohs hardness value of not less than 5 are deposited at a density of 500 to 200,000 particles/cm.sup.2, and a second roll having smooth surface, said first and second rolls being arranged to oppose each other;
• and
• pressure control means for adjusting a pressure applied to an elongated organic film which is fed between said first and second rolls, the method comprising the steps of:
• feeding the elongated organic film between said first and second rolls;
• adjusting a force acting on the elongated organic film between said first and second rolls by said pressure control means to be uniform over the entire surface which is brought into contact with each of said first and second rolls;
• rotating said first and second rolls in opposite directions to pass the elongated organic film between said first and second rolls and causing said sharp pointed portions of said particles deposited on the surface of said first roll to press into the elongated organic film, thereby forming non-through pores in the elongated organic film at a density of 500 to 200,000 pores/cm.sup.2.