To provide a sleeve for preventing concrete from entering a sleeve, and easily removing the sleeve from a concrete skeleton.

This sleeve includes: a cylindrical member 7 formed of a flexible sheet member 5 cylindrically wound to reduce the diameter to a small diameter than a piping hole 4 and having an overlap part 6; an enlarged diameter core part 9, which is formed of material having elasticity, partly or entirely fitted to an opening part 8a of the cylindrical member 7, thereby holding the diameter of the opening part 8a in a predetermined shape against the energizing force in the diameter reducing direction of the cylindrical member 7; and a flange part 10, which is formed on the outer periphery of the opening part 8a side end of the cylindrical member 7 in the enlarged diameter core part 9, and prevents concrete from entering the interior of the cylindrical member 7 through a gap between the opening part 8a and the concrete form 3.