The critical spontaneous ignition temperature for activated carbon were observed by performing experiments for granulated activated carbon with varying the period. As the results of the experiments, the critical spontaneous ignition temperature according to period of 0.5hr, 1.5hr and 2.5hr was 177.5$^{circ}C$, 176.5$^{circ}C$ and 172.5$^{circ}C$ respectively, and the approximate expression of the critical spontaneous ignition temperature obtained for ambient temperature amplitudes was as follow ; $delta$$_{c}$ (X, $omega$)=$delta$$_{c}$ ｛1-0.381 exp(-0.930.$omega$$^{0.811}$)｝)｝