A method including the steps of assembling a battery pack core; holding the core in a mold that forms a molded resin region; closing a cavity of the mold; injecting synthetic resin into the closed cavity; and ejecting the battery pack from the cavity to configure the battery pack with the battery pack core insert-molded in the molded resin region. A projection that protrudes towards an inner surface of the mold cavity is provided on the surface of the insulating cover fixed on the battery. In the resin injection step, the projection on the insulating cover is pressed directly or indirectly by the mold in a direction to put the insulating cover tightly in contact with the surface of the battery; and synthetic resin is injected into the cavity of the mold in this pressed configuration.