A joint structure connecting a first and second double pipe, includes a male bayonet at an end portion of one of the pipes, the male bayonet including a first flange; a female bayonet at an end portion of the other of the pipes, the female bayonet including a second flange fastened to the first; 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 so that a center axis of the male bayonet and the female bayonet conform to each other; and a slide mechanism which supports the first positioning member so the first positioning member is slidable in an axial direction of the first double pipe or supports the second positioning member so the second positioning member is slidable in an axial direction of the second double pipe.