Provided is a reactive polymer-supporting porous film for use as a battery separator which comprises a porous substrate film and a partially crosslinked reactive polymer supported on the porous substrate film, the partially crosslinked reactive polymer being obtained by the reaction of a crosslinkable polymer having at least one reactive group selected from the group consisting of 3-oxetanyl group and epoxy group in the molecule with a monocarboxylic acid. Further, provided is a method of manufacturing a battery which comprises layering electrodes on the reactive polymer-supporting porous film.