PURPOSE: To be able to bore a hole in a battery after assembling by covering the wall surface of a bored hole with a thermoplastic resin film when a sheet sheet battery was bored, fitting a thermoplastic resin cylinder in the bored hole, inserting a horn of an ultrasonic welder into the resin cylinder, and melt bonding the film and resin cylinder.

CONSTITUTION: When a sheet battery is bored with a drill 10, a resin film 108 is formed on the inner surface of a bored hole 12. A thermoplastic resin cylinder 106 having almost the same diameter as the hole 12 is inserted into the hole 12, then an ultrasonic horn 109 of an ultrasonic welder 107 is inserted into the cylinder 106, and ultrasonic welding is conducted while the horn 109 is gradually drawn out. The resin film 108 is welded to resin films 101a, 101b, which are laminated on both sides of the sheet battery, and the cylinder 106, and the bored hole surface of the sheet battery is sealed so as to keep an insulating condition.