Abstract: A complex oxide comprising zirconium oxide and an oxide of at least one metal selected from the group consisting of an alkali metals, alkaline earth metals, silicon and lead is mixed with pottery stone, magnesium fluoride, a compound capable of forming magnesium oxide, a compound capable of forming potassium oxide and a compound capable of forming boron oxide to obtain a composition in which the contents of elements in 100 parts by weight of the total amount exclusive of the ignition loss are 16 to 25 parts of silicon, 6 to 9 parts of aluminum, 5.5 to 12 parts of potassium, 4.7 to 15 parts of magnesium, 0.7 to 5.0 parts of zirconium, 0.7 to 3.2 parts of boron, 3.0 to 9.3 parts of fluorine, up to 7.0 parts of metals other than the foregoing elements, and the balance being oxygen. This composition is heat-melted and then cooled from a temperature higher at least 50.degree. C. than the glass transition temperature of a vitreous body to be formed to a temperature lower by at least 70.degree. C.