PROBLEM TO BE SOLVED: To provide a grout outflow preventive sealant between an inner cylindrical pipe and an outer cylindrical pipe, and a seal structure thereof, for easily changing a structure, a shape and a thickness, depending on a condition such as a placing height and a diameter of a grout material, excellent in versatility without requiring to manufacture by using an exclusive mold, and easily constructible without requiring to apply a large load for installation of the sealant.SOLUTION: Both end parts 10b in a width direction of an elastic raw material 10a such as a plate-like rubber-synthetic resin, are folded back, and both end parts 10c in the longitudinal direction orthogonal to this are joined, and are formed in a double ring shape 10d, and a folding-back part 10e of an elastic body of this double ring shape is installed by being extended along an inner peripheral surface 2a of an outer cylindrical pipe 2, and a filling fluid 12 is pressurized and filled inside the double ring shape, thereby a se
