To solve problems that presently a void pipe without having draft angle nor flexibility is used for a form for an opening part provided on every floor for the purpose of equipment-piping in a reinforced concrete high-storied building, which turns out to require quite much labor in mounting it as well as in removing the form after concrete is solidified and that it cannot be diverted because of being broken on that occasion and thus is disposed as an industrial waste while in a series of plastering work, since it is projected from a finished surface of a slab, it becomes hindrance for the work and thus work efficiency is decreased.

In order to remove the opening form easily and without damaging from the hardened concrete, a polyvinyl chloride pipe having a notch in a trunk part of a cylindrical shape in the axial direction for allowing self-contraction property is used. Accordingly, the divertible opening form can easily be disassembled while at the same time the total height of the form is a little lower that the thickness of the slab, so as not to be an obstacle in a series of plastering work.