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In order to determine the fracture toughness of rocks at high temperature, splitting tests were conducted on KIMACHI sandstone and INADA granite in the electric furnace. The rock specimens (200 mm ×150 mm ×20 mm) that have a slot are prepared in this test. The slot induces stress concentration around the tip of slot when splitting load is applied to the slot, and fracture initiation occurs form the tip of slot into the specimen. Crack initiation and process of crack propagation are investigated by the measurement of splitting load, crack opening displacement and crack length. Stress intensity factors during crack propagation are calculated with the compliance calibration, and the fracture toughness KIC of rock at heigh temperature are determined. From the study of the effect of temperature on the fracture toughess, it becomes clear that two tendencies are seen in increase and decrease of the fracture toughness by heating. The fracture toughness of clastic rock such as KIMACHI sandstone increases with rising temperature, though once it decreases at 100℃. For instance, the fracture toughness at 300℃ is about 1.4 times larger than that at the room temperature. On the other hand, in the case of crystaline rock such as INADA granite, the fracture toughness decreases with rising temperature. For instance, compared with the fracture toughness at 300℃ and that at the room temperature, decrease of the fracture toughness is about 35%.