An assembly jig includes an upper jig frame that is provided above a longitudinal outer edge of a large structure, a lower jig frame that is provided below a longitudinal outer edge of the large structure, a connecting jig frame that connects the upper jig frame and the lower jig frame to each other, three supporting parts for supporting the lower jig frame from below, and a jig leg portion that is provided perpendicular to the direction in which the lower jig frame extends and parallel to a ground surface. The supporting parts are provided at three portions that are respectively located under two end portions of the jig leg portion and under the lower jig frame, and arrangement positions of the supporting parts provided at the three portions form a substantially triangular shape when seen from above.