A battery pack including: a box main body housing a battery therein and having a box shape with an upper side opening; and a lid covering the upper side opening of the box main body. The box main body has a first seal flange formed along an outer peripheral portion of the box main body, and the lid has a second seal flange formed along an outer peripheral portion of the lid. The first and second seal flanges are in contact with each other and seal the battery pack. The first and second seal flanges are curved with a predetermined curvature.