Conversion of used glass bottles to porous materials was investigated for recycling of used glasses. The ordinary method to prepare porous materials from glass needs vesicants such as calcium carbonate and silicon carbide that decompose at high temperatures to produce gas, and the gas is trapped in softened glass to form pores. In this study, used glass bottles were converted to porous plates by using hydrothermal technique. First, a glass powder is hydrothermally treated at low temperatures, so that water diffuses into the glass structure. Water is released as vapor to form pores in the softened glass, when the hydrothermally treated glass powder is heated at high temperatures. Thus, this process gives porous materials with a fine structure including closed pores at lower temperature than the ordinary method. The effects of hydrothermal treatment and calcination conditions on bulk density of the resulted porous materials were investigated with a small scale to determine the conditions to get light porous materials. Finally, porous plates (45x45x3 cm3 in size) with density of 0.45g/cm3 were produced by hydrothermal treatment of the glass powder at 180°C in a large autoclave with inside volume of 2.5 m3, followed by calcination at 800°C in a continuous heater with 18 m in length.