Volatilization of I34Cs from simulated high-level waste glass in a canister during several reheatings up to a maximum of 1000°C was examined. The results showed that the temperature dependence of the amount of 134Cs suspended in the air inside the canister could be divided into two categories. As the temperature was increased above 500°C, the amount of 134Cs suspended in the air inside the canister also increased. On the other hand, for temperatures <500°C, the amount of 134Cs suspended in the air inside the canister had an almost constant value after several reheatings up to a maximum of 1000°C. In this case, the air contamination by cesium-bearing material inside the canister is considered to be significant even at waste storage temperatures <500°C.