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 (S) 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 , <EOS>