Abstract: A foam glass is manufactured by mixing silicate glass, a volcanic glass material (volcanic ashes of a special chemical composition, occurring in the Kyushu District of Japan), water glass and at least one member selected from the group consisting of sodium salts and calcium salts of phosphoric acid, firing the resultant mixture into a fused state and cooling the fused mixture. The foam glass has a high aluminum content, contains numerous fine bubbles less than 1.5mm in diameter, has a bulk density of less than 1 and shows excellent mechanical strength such as, for example, high bending strength.