A composite including an electrolyte layer containing solid oxide, and at least one electrode selected from a cathode disposed on one side of the electrolyte layer in a first direction and an anode disposed on the other side of the electrolyte layer in the first direction. Either one of two surfaces of the composite located on opposite sides in the first direction satisfies a first requirement that, as viewed in the first direction, a curvature determined on the basis of any three points juxtaposed at intervals of 5 mm is less than 0.0013 (l/mm) and that, as viewed in a second direction perpendicular to the first direction, the curvature is the reciprocal of the radius of an imaginary circle passing through the any three points.