In a canister horizontally installed and housed inside a concrete silo, at least two temperatures out of a temperature TB at a canister bottom portion to be one end portion in a lateral direction in a horizontally-installed attitude, a temperature TSB at a canister side surface lower portion located below a horizontal plane passing through a center of the canister, a temperature TT at a canister lid portion to be the other end portion in the lateral direction, and a temperature TST at a canister side surface upper portion located above the horizontal plane passing through the center of the canister are monitored, and occurrence of leakage of an inert gas inside the canister is detected when there is a change in a temperature difference between the at least two temperatures.