A first flange and a second flange are superposed and fastened with a bolt and a nut in a fastening operation, to implement a fastening structure used at temperature lower than atmospheric temperature for the fastening operation. Assuming that the first flange has a thickness L1 and a coefficient of linear expansion α1, the second flange has a thickness L2 and a coefficient of linear expansion α2, superposed first flange and second flange have a thickness L, and the bolt has a coefficient of linear expansion α, a relation of L·α≧L1·α1+L2·α2 is satisfied, i.e. heat shrinkage of the bolt is not smaller than the total heat shrinkage of the first and second flanges. Thus, impairment of sealing property due to heat shrinkage is avoided at a flange junction used at low temperature.