A battery pack comprising: a cylindrical battery; a circuit board that is electrically connected to the cylindrical battery and is mounted with a charge and discharge circuit of the battery pack; and an exterior case including a first case and a second case, the exterior case housing the cylindrical battery and the circuit board, the first case including: a curved portion that curves along an outer peripheral surface of the cylindrical battery; and a first vertical wall that is integrally connected to an end edge of the curved portion, the second case including a second vertical wall connected to the first vertical wall, the first vertical wall having a fitting protrusion protruding from an opposed surface facing the second vertical wall, the second vertical wall having a fitting groove, the fitting groove being disposed in an opposed surface facing the first vertical wall and guiding the fitting protrusion, the first vertical wall and the second vertical wall being welded together with the fitting protrusion being guided into the fitting groove, and the first vertical wall having an inner surface in a concave shape along a surface of the cylindrical battery, and a leading end portion with the opposed surface, being thicker than the curved portion, and the opposed surface of the first vertical wall comprises a stepped portion outside the fitting protrusion, wherein the opposed surface of the first vertical wall is divided into a first opposed surface (llx) and a second opposed surface (lly) by providing the stepped portion outside the fitting protrusion, and wherein the first opposed surface (llx) constitutes a leading end surface and is connected to the fitting protrusion, and the second opposed surface (lly) is a stepped surface formed one step lower than the first opposed surface (llx) and forms a gap between the first vertical wall and the second vertical wall when the fitting protrusion is inserted into the fitting groove. a cylindrical battery; a circuit board that is electrically connected to the cylindrical battery and is mounted with a charge and discharge circuit of the battery pack; and an exterior case including a first case and a second case, the exterior case housing the cylindrical battery and the circuit board, the first case including: a curved portion that curves along an outer peripheral surface of the cylindrical battery; and a first vertical wall that is integrally connected to an end edge of the curved portion, a curved portion that curves along an outer peripheral surface of the cylindrical battery; and a first vertical wall that is integrally connected to an end edge of the curved portion, the second case including a second vertical wall connected to the first vertical wall, the first vertical wall having a fitting protrusion protruding from an opposed surface facing the second vertical wall, the second vertical wall having a fitting groove, the fitting groove being disposed in an opposed surface facing the first vertical wall and guiding the fitting protrusion, the first vertical wall and the second vertical wall being welded together with the fitting protrusion being guided into the fitting groove, and the first vertical wall having an inner surface in a concave shape along a surface of the cylindrical battery, and a leading end portion with the opposed surface, being thicker than the curved portion, and the opposed surface of the first vertical wall comprises a stepped portion outside the fitting protrusion,