Method for producing porous polyimide film, porous polyimide film and separator using same

A method for producing a porous polyimide film, the method comprising:

• a first unburned composite film-forming step of forming a first unburned composite film of a first varnish on a substrate, the first varnish containing a polyamide acid or polyimide (A1) and fine particles (B1) at a volume ratio (A1):(B1) of 19:81 to 45:65;
• a second unburned composite film-forming step of forming a second unburned composite film of a second varnish on the first unburned composite film, the second varnish containing a polyamide acid or polyimide (A2) and fine particles (B2) at a volume ratio (A2):(B2) of 20:80 to 50:50 and having a fine particle content ratio lower than that of the first varnish;
• a burning step of burning an unburned composite film composed of the first unburned composite film and the second unburned composite film to prepare a polyimide-fine particle composite film;
• and
• a fine particle-removing step of removing the fine particles from the polyimide-fine particle composite film.