A square battery, comprising: - a battery can surrounded by four side wall portions (100a, 100b), and having an upper end portion on one side in a height direction of the side wall portions (100a, 100b) opened and having a bottom portion in a lower end portion on the other side in the height direction, and - a battery lid welded to the upper end portion of the battery can to seal the battery can (100, wherein between the battery can and the battery lid, a lateral boundary surface (Fx) in a lateral direction crossing the height direction of the side wall portions and a vertical boundary surface (Fy) in a vertical direction crossing the lateral boundary surface and along the height direction of the side wall portions are formed, the battery can has an upper end surface in the lateral direction crossing the height direction of the side wall portions (100a, 100b), the upper end surface being provided at the upper end portion of the battery can, an inner wall surface in the vertical direction along the height direction of the side wall portions (100a, 100b), the inner wall surface crossing the upper end surface and being provided inside the battery can, and an outer wall surface in the vertical direction along the height direction of the side wall portions (100a, 100b), the outer wall surface being provided on the outer side of the battery can, and the upper end surface being formed bridging over the inner wall surface and the outer wall surface, and the battery lid has a first opposed surface forming a lateral boundary surface (Fx) so as to be opposed to the upper end surface of the battery can, and a second opposed surface crossing the first opposed surface and opposed to the inner wall surface of the battery can to form a vertical boundary surface (Fy), characterised in that: the battery lid has a flange portion provided along an outer circumference portion of the battery lid and upper projection portion which protrudes in a thickness direction of the battery lid rather than the flange portion and toward a side separating from the battery can, and whereinthe battery lid has a protrusion portion which protrudes in the same direction as the upper projection portion from the flange portion and is melted by the laser, wherein the lateral boundary surface (Fx) is partly welded from a corner at which the lateral boundary surface (Fx) and the vertical boundary surface (Fy) cross with each other by a prescribed welding width, and the vertical boundary surface (Fy) is partly welded from the corner by a prescribed welding depth. - a battery can surrounded by four side wall portions (100a, 100b), and having an upper end portion on one side in a height direction of the side wall portions (100a, 100b) opened and having a bottom portion in a lower end portion on the other side in the height direction, and - a battery lid welded to the upper end portion of the battery can to seal the battery can (100, wherein between the battery can and the battery lid, a lateral boundary surface (Fx) in a lateral direction crossing the height direction of the side wall portions and a vertical boundary surface (Fy) in a vertical direction crossing the lateral boundary surface and along the height direction of the side wall portions are formed, the battery lid has a flange portion provided along an outer circumference portion of the battery lid and upper projection portion which protrudes in a thickness direction of the battery lid rather than the flange portion and toward a side separating from the battery can, and whereinthe battery lid has a protrusion portion which protrudes in the same direction as the upper projection portion from the flange portion and is melted by the laser, wherein the lateral boundary surface (Fx) is partly welded from a corner at which the lateral boundary surface (Fx) and the vertical boundary surface (Fy) cross with each other by a prescribed welding width, and the vertical boundary surface (Fy) is partly welded from the corner by a prescribed welding depth.