Provided is a seal structure, according to the present invention, for sealing opposite surfaces of flanges between adjacent tail ducts, which can be prevented from being worn or aged deteriorated due to a thermal deformation in a high temperature atmosphere or vibration of a gas turbine combustor, and which can maintain a satisfactory sealing function for a long time. The seal structure is characterized in that recess grooves are formed in opposed surfaces of adjacent flanges at the outlet end of tail ducts, striding between the opposed surfaces, a seal assembly composed of a seal member and a leaf spring is inserted in the recess grooves, the seal member has a pair of continuous beads which are arranged facing the inner surfaces, on the combustion gas passage side, of the recess grooves, striding between the opposed surfaces of the flanges within the recess grooves, the leaf spring is arranged so as to be contact with the seal member within the recess grooves so that the beads are pressed against the surfaces on the combustion gas passage side by the resilient force of the leaf spring so as to create seal surfaces.