To provide a sleeve for forming piping holes different in diameter using the same member, and further 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 the piping holes 4 and having an overlap part 6; and an enlarged diameter core part, which is partly or entirely fitted to an opening part 8a of the cylindrical member 7, thereby holding the diameter of the opening part 8a to be enlarged or reduced against the energizing force in the diameter reducing direction of the cylindrical member 7.