A welding structure characterized by comprising: a sheet-shaped first part to be welded (11; 111; 211; 311); a sheet-shaped second part to be welded (21; 121; 221; 321) that is positioned with respect to the first part to be welded (11; 111; 211; 311) by engaging with the first part to be welded (11; 111; 211; 311), in order to weld the second part to be welded (21; 121; 221; 321) to the first part to be welded (11; 111; 211; 311); and a first positioning portion, a second positioning portion, charcterized by a third positioning portion, by which the first part to be welded (11; 111; 211; 311) engages with the second part to be welded (21; 121; 221; 321), and characterized in that the first positioning portion, the second positioning portion, and the third positioning portion are each configured to restrict movement of one of the first part to be welded (11; 111; 211; 311) and the second part to be welded (21; 121; 221; 321) in a positioning direction that is one direction, with respect to the other of the first part to be welded (11; 111; 211; 311) and the second part to be welded (21; 121; 221; 321), and enable movement of the one of the first part to be welded (11; 111; 211; 311) and the second part to be welded (21; 121; 221; 321) in a movable direction that is perpendicular to the positioning direction, with respect to the other of the first part to be welded (11; 111; 211; 311) and the second part to be welded (21; 121; 221; 321); and the first positioning portion, the second positioning portion, and the third positioning portion are arranged such that the movable direction of each of the first positioning portion and the second positioning portion is the same direction as the positioning direction of the third positioning portion, and in that the first positioning portion, the second positioning portion, and the third positioning portion are arranged such that a distance between the first positioning portion and the second positioning portion in the movable direction is greater than a distance between the first positioning portion and the second positioning portion in the positioning direction, wherein a distance between the first positioning portion and the second positioning portion is greater than whichever is smaller of a distance between the first positioning portion and the third positioning portion, and a distance between the second positioning portion and the third positioning portion. a sheet-shaped first part to be welded (11; 111; 211; 311); a sheet-shaped second part to be welded (21; 121; 221; 321) that is positioned with respect to the first part to be welded (11; 111; 211; 311) by engaging with the first part to be welded (11; 111; 211; 311), in order to weld the second part to be welded (21; 121; 221; 321) to the first part to be welded (11; 111; 211; 311); and a first positioning portion, a second positioning portion, charcterized by a third positioning portion, by which the first part to be welded (11; 111; 211; 311) engages with the second part to be welded (21; 121; 221; 321), and characterized in that the first positioning portion, the second positioning portion, and the third positioning portion are each configured to restrict movement of one of the first part to be welded (11; 111; 211; 311) and the second part to be welded (21; 121; 221; 321) in a positioning direction that is one direction, with respect to the other of the first part to be welded (11; 111; 211; 311) and the second part to be welded (21; 121; 221; 321), and enable movement of the one of the first part to be welded (11; 111; 211; 311) and the second part to be welded (21; 121; 221; 321) in a movable direction that is perpendicular to the positioning direction, with respect to the other of the first part to be welded (11; 111; 211; 311) and the second part to be welded (21; 121; 221; 321); and the first positioning portion, the second positioning portion, and the third positioning portion are arranged such that the movable direction of each of the first positioning portion and the second positioning portion is the same direction as the positioning direction of the third positioning portion, and in that the first positioning portion, the second positioning portion, and the third positioning portion are arranged such that a distance between the first positioning portion and the second positioning portion in the movable direction is greater than a distance between the first positioning portion and the second positioning portion in the positioning direction, wherein a distance between the first positioning portion and the second positioning portion is greater than whichever is smaller of a distance between the first positioning portion and the third positioning portion, and a distance between the second positioning portion and the third positioning portion.