A double tube for welded structure that comprises an inside tube of a heat resistant ferritic steel having chemical composition 1 described below and an outside tube of a heat resistant austenitic steel having chemical composition 2 described below, and is to be used to produce a welded structure by means of forming of a fillet weld zone satisfying Formula described below on an outside surface thereof, wherein wall thicknesses (mm) of the outside tube and inside tube satisfy Formula described below, wherein wall thickness of outside tube ≥ fusion depth in fillet weld + 0.3 mm wall thickness of outside tube / ( wall thickness of outside tube + wall thickness of inside tube) ≤ 0.4 and wherein Chemical composition 1 is a chemical composition comprising, by mass percent, C: 0.01 to 0.1%, Si: 0.01 to 0.5%, Mn: 0.01 to 1%, P: 0.03% or less, S: 0.01% or less, one or more kinds of Ni and Cu: 0.01 to 1% in total, Cr: 0.5 to 3.5%, one or more kinds of Mo and W: 0.01 to 3% in total, V: 0.01 to 0.5%, Nb: 0.005 to 0.1%, Ti: 0.0005 to 0.1%, B: 0.0001 to 0.02%, Al: 0.05% or less, N: 0.0005 to 0.05%, O: 0.01% or less, and optionally one or more kinds of elements selected from Ca: 0.05% or less, Mg: 0.05% or less, and REM: 0.1% or less, the balance being Fe and impurities, and wherein Chemical composition 2 is a chemical composition comprising, by mass percent, C: 0.1% or less, Si: 0.01 to 0.8%, Mn: 0.01 to 2%, P: 0.04% or less, S: 0.01% or less, Ni: 5 to 50%, Cr: 15 to 35%, Al: 0.05% or less, N: 0.001 to 0.25%, O: 0.01% or less, and optionally one or more kinds of elements shown in the first group to fifth group described below, the balance being Fe and impurities, wherein The first group is: one or more kinds selected from Mo: 10% or less and W: 10% or less The second group is: one or more kinds selected from Cu: 5% or less and Co: 5% or less The third group is: one or more kinds selected from Nb: 1% or less, Ti: 1% or less, and V: 1% or less The fourth group is: B: 0.02% or less The fifth group is: one or more kinds selected from Ca: 0.05% or less, Mg: 0.05% or less, and REM: 0.1% or less. Chemical composition 1 is a chemical composition comprising, by mass percent, C: 0.01 to 0.1%, Si: 0.01 to 0.5%, Mn: 0.01 to 1%, P: 0.03% or less, S: 0.01% or less, one or more kinds of Ni and Cu: 0.01 to 1% in total, Cr: 0.5 to 3.5%, one or more kinds of Mo and W: 0.01 to 3% in total, V: 0.01 to 0.5%, Nb: 0.005 to 0.1%, Ti: 0.0005 to 0.1%, B: 0.0001 to 0.02%, Al: 0.05% or less, N: 0.0005 to 0.05%, O: 0.01% or less, and optionally one or more kinds of elements selected from Ca: 0.05% or less, Mg: 0.05% or less, and REM: 0.1% or less, the balance being Fe and impurities, and wherein Chemical composition 2 is a chemical composition comprising, by mass percent, C: 0.1% or less, Si: 0.01 to 0.8%, Mn: 0.01 to 2%, P: 0.04% or less, S: 0.01% or less, Ni: 5 to 50%, Cr: 15 to 35%, Al: 0.05% or less, N: 0.001 to 0.25%, O: 0.01% or less, and optionally one or more kinds of elements shown in the first group to fifth group described below, the balance being Fe and impurities, wherein The first group is: one or more kinds selected from Mo: 10% or less and W: 10% or less The second group is: one or more kinds selected from Cu: 5% or less and Co: 5% or less The third group is: one or more kinds selected from Nb: 1% or less, Ti: 1% or less, and V: 1% or less The fourth group is: B: 0.02% or less The fifth group is: one or more kinds selected from Ca: 0.05% or less, Mg: 0.05% or less, and REM: 0.1% or less. The first group is: one or more kinds selected from Mo: 10% or less and W: 10% or less The second group is: one or more kinds selected from Cu: 5% or less and Co: 5% or less The third group is: one or more kinds selected from Nb: 1% or less, Ti: 1% or less, and V: 1% or less The fourth group is: B: 0.02% or less The fifth group is: one or more kinds selected from Ca: 0.05% or less, Mg: 0.05% or less, and REM: 0.1% or less.