A reactive polymer-supporting porous film is provided 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 is obtained by the reaction of a crosslinkable polymer having at least one group selected from the 3-oxetanyl group and epoxy group reactive with a polycarboxylic acid. The reactive polymer-supporting porous film has a separator and electrodes sufficiently bonded to each other and with low inner resistance suitably used for production of battery excellent in high rate performance. Further, provided is a method of producing a battery which comprises placing the electrode/reactive polymer-supporting porous film layered body in a battery container; introducing an electrolytic solution containing a cationic polymerization catalyst into the battery container thereby bonding the porous film and electrodes together.