To provide an expansible sleeve capable of minimizing the influence of a progressing condition of reinforcement assembly work and efficiently carrying out installation work by a small number of workers.

An inside bending prevention sleeve 1 is constituted of a slide void outside 2, a slide void inside 3 and an inside bending prevention ring 4 of the slide void inside, and both inside and outside slide voids are stored inside of the inside bending prevention sleeve in a movably expansible manner. The inside bending prevention sleeve is inserted into a beam bar in a shranken state, it is expanded into the width of a form, and after it has been temporarily fastened with a double faced adhesive tape stuck on caps 5 and 6 inserted into both ends, it is fixed with a nail.