PURPOSE: To treat a high-temperature medium that remains at a primary side without causing damage to the environment and facilities even if a cooling pipe system fails by providing at least one overlapping cooling pipe system at a vapor generator in the vapor generator with the cooling pipe system for diffusion heat at a secondary side.

CONSTITUTION: A high-temperature medium such as a flue gas from a carbonization gas combustion facility is supplied to a vapor generator 1 through an inflow pipe port 1a. Two overlapped cooling pipe systems 2 and 3 are provided in the vapor generator 1 and are connected each other by a combination crosspiece 6 with heat transfer property. The cooled medium passes through an outflow pipe port 1b and is unloaded from the vapor generator 1. Then, both the cooling pipe systems 2 and 3 are connected, for example, by a steam turbine. When either of the cooling pipe systems 2 and 3 fails, the other cooling pipe can be utilized successively for cooling, thus preventing a facility part such as the vapor generator from being damaged and a harmful substance from being discharged into the atmosphere.