This battery pack is assembled by welding an opening portion (20) of a waterproof bag (2) to provide an insertion opening (23) that is smaller than the total opening width (W) but allows a battery core pack (1) to be inserted, inserting the battery core pack (1) into the waterproof bag (2), then, closing the opening portion (20) of the waterproof bag (2), and placing the waterproof bag (2), in which the battery core pack (1) has been placed, into an external case (3). In this way, this battery pack can be assembled efficiently, with the battery core pack having been given a waterproof structure by the waterproof bag (2) while in an ideal state.