Changes in pore spaces and microcracks in Shirahama sandstone subjected to slow and uniform temperature changes up to 1,000 °C have been examined using a fluorescent technique. A specimen heated up to a certain temperature was kept at the constant for 24 hours, and then cooled down to a room temperature. Texture of the specimen was observed with a microscope. <br> Above 500 °C, new microcracks were observed along grain boundaries. The widening of grain boundary microcracks was recognized with increasing temperature. Thermal expansion caused by the α to β transition of quartz plays the dominant role in the development of grain boundary microcracks. Above 500 ^°C, increase and enlargement of pore spaces in the matrix were also observed. The changes in matrix are caused by thermal reactions in the matrix constituent materials such as dehydration and formation of new minerals. At high temperatures, these reactions and the quartz expansion may occur simultaneously, so that microcracks and pore spaces significantly increase. Changes in physical properties such as porosity and elastic wave velocity show good agreement with the results obtained by microscopic observations.<br>