PURPOSE: To prevent penetration of water from outside and to eliminate damage caused by freezing, by forming water-repellent covering layer on the surface of expanded glass material.

CONSTITUTION: A water-repellent covering layer is formed on least one surface of expanded glass material having a large number of closed cells in the interior, consisting of glass. Rubber, thermoplastic resin, or thermosetting resin is used as the covering layer. Covering is carried out by any of coating with brushes, rollers, etc., spraying method by mortar gun, spray gun, etc., and dipping method, and an amount of the covering material used is 5W500mg based on 1cm2.