To provide void members that are durable to breakage/deformation upon pouring concrete, can accurately form a penetrating hole with designed diameter, and can be easily removed from the cured concrete.

A void member 3, which is used for forming a penetrating hole 1a in a concrete wall 1, consists of an external member 6 and an inner member 7. The external member 6 has a cylindrical form and an inserting hole 6a inside. The inner member 7 reinforces the external member 6 by being removably inserted into the hole 6. The external member 6 is made of expandable synthetic resin that can be removed by scraping off, cutting off, tearing up, heating to melt, or burning off within the penetrating hole 1a. By removing the inner member 7 from the external member 6 after the concrete 1b is placed and cured, a working space S during removing the external member 6 from the concrete wall 1 is formed with in the external member 6.