Ajoint structure (1A, 1B) connecting a first double pipe and a second double pipe to each other, the joint structure comprising: a male bayonet provided at an end portion of one of the first double pipe and the second double pipe, the male bayonet including a first flange and a first tubular body protruding from the first flange in an axial direction of the one of the first double pipe and the second double pipe; a female bayonet provided at an end portion of the other of the first double pipe and the second double pipe, the female bayonet including a second flange to be fastened to the first flange, an inner housing, and an outer housing, the second flange being coupled to an outer pipe of the other of the first double pipe and the second double pipe via the outer housing; a first positioning member mounted on the first double pipe; a second positioning member mounted on the second double pipe, the second positioning member being positioned with respect to the first positioning member so that a center axis of the male bayonet and a center axis of the female bayonet conform to each other; and a slide mechanism (8, 9) which supports the first positioning member so that the first positioning member is slidable in an axial direction of the first double pipe or supports the second positioning member so that the second positioning member is slidable in an axial direction of the second double pipe; characterised in that the female bayonet includes a second tubular body, the second flange being coupled to an inner pipe of the other of the double pipes via the inner housing and the second tubular body, and in that the female bayonet has an insertion opening which is provided between the second tubular body and the inner housing and into which the first tubular body is insertable. a male bayonet provided at an end portion of one of the first double pipe and the second double pipe, the male bayonet including a first flange and a first tubular body protruding from the first flange in an axial direction of the one of the first double pipe and the second double pipe; a female bayonet provided at an end portion of the other of the first double pipe and the second double pipe, the female bayonet including a second flange to be fastened to the first flange, an inner housing, and an outer housing, the second flange being coupled to an outer pipe of the other of the first double pipe and the second double pipe via the outer housing; a first positioning member mounted on the first double pipe; a second positioning member mounted on the second double pipe, the second positioning member being positioned with respect to the first positioning member so that a center axis of the male bayonet and a center axis of the female bayonet conform to each other; and a slide mechanism (8, 9) which supports the first positioning member so that the first positioning member is slidable in an axial direction of the first double pipe or supports the second positioning member so that the second positioning member is slidable in an axial direction of the second double pipe; characterised in that the female bayonet includes a second tubular body, the second flange being coupled to an inner pipe of the other of the double pipes via the inner housing and the second tubular body, and in that the female bayonet has an insertion opening which is provided between the second tubular body and the inner housing and into which the first tubular body is insertable.