A double tube for welded structure comprises an inside tube of a heat resistant ferritic steel and an outside tube of a heat resistant austenitic steel. The inside tube has a chemical composition which comprises specific amounts of C, Si, Mn P, S, Ni + Cu, Cr, Mo + W, V, Nb, Ti, B, Al, N and O with the balance of Fe and impurities. The outside tube has a chemical composition which comprises specific amounts of C, Si, Mn, P, S, Ni, Cr, Al, N and O with the balance of Fe and impurities. A fillet weld zone, which satisfies Formula of [wall thickness of outside tube ‰¥ fusion depth in fillet weld + 0.3 mm], is formed on the outside surface of the double tube, and the wall thicknesses (mm) of the outside tube and inside tube satisfy Formula of [wall thickness of outside tube / (wall thickness of outside tube + wall thickness of inside tube) ‰¤ 0.4]. The double tube is excellent in weld crack resistance. If this double tube is used, a welded structure forming various types of high-temperature equipment can be manufactured by fillet-welding plates, metal fittings, and the like to the outside surface of the tube by means of ordinary submerged arc welding without performing treatment such as preheating, postheating, and cleaning of steel. The inside tube may contain one or more kinds of elements selected from Ca, Mg and REM, and the outside tube may contain one or more kinds of elements selected from Mo, W, Cu, Co, Nb, Ti, V, B, Ca, Mg and REM.