NOVELTY - The material (10) has a double annular elastic body i.e. rubber, whose folding part (10e) is extended along an internal peripheral surface (2a) of an outer cylinder pipe i.e. sheath (2). Filling fluid (12) is pressurized in an inner side of the double annular elastic body. An expansion gap (d) is formed in a seal state such that holding of the annular elastic body is possible. A through-hole (10f) is connected with a stud bolt (13) that is fixed in the internal peripheral surface of the outer cylinder pipe. The outer cylinder pipe is made of a flat elastic raw material.    USE - Sealing material e.g. lip-type sealing material and filling type sealing material, for preventing grout escape between an inner cylinder pipe i.e. pile, and an outer cylinder pipe i.e. sheath, during construction of a marine windmill for wind power generation.    ADVANTAGE - The expansion gap is formed in the seal state such that holding of the annular elastic body is possible so as to change aperture diameter, shape, thickness and placement height of the grout material in an easy manner and improve versatility. The material prevents outflow of the grout material poured into the expansion gap between the inner cylinder pipe and the outer cylinder pipe. The material reduces usage amount of welding materials.    DETAILED DESCRIPTION - The flat elastic raw material is flat rubber and synthetic resin. An INDEPENDENT CLAIM is also included for a seal structure for grout escape prevention between an inner cylinder pipe and an outer cylinder pipe.    DESCRIPTION OF DRAWING(S) - The drawing shows a cross-sectional view of a sealing material for preventing grout escape between an inner cylinder pipe and an outer cylinder pipe at an attachment state to a sheath.    Expansion gap (d)    Sheath (2)    Internal peripheral surface of an outer cylinder pipe (2a)    Sealing material (10)    Folding part of a double annular elastic body (10e)    Through-hole (10f)    Filling fluid (12)    Stud bolt (13)