A heap of a material to be leached to recover a valuable metal from the material includes an electromagnetic heating system to generate heat in situ in the heap. Moreover, thickness of joining layer decreases in at least a part of joining layer in direction from inflow end surface side toward outflow end surface side, and length L2 of joining layer in axial direction X is smaller than 95% of length L1 of honeycomb segment in axial direction X. Optimal radiopacity is achieved when the end sets of strut members are more radiopaque as compared to the radiopacity of the central sets of strut members. Two transmission lines connected to the output terminals at both ends have a third transmission line which is connected in series and a fourth transmission line whose electrical length is \xbc wave length respectively. During cleaning of the throttle valve, the CVD chamber is isolated, by closing the chamber isolation valve, and cleaning gases are flowed into the throttle valve, by opening the cleaning isolation valve.