PURPOSE: To obtain a steam-generating device for increasing the efficiency of the device by decreasing thermal loss.

CONSTITUTION: A steam-generating device 10 includes a reactor 12 and a separator 14. In a duct 16, the rear wall of the reactor 12 is connected to a front wall 14a of the separator 14, the wall of the duct 16 is isolated, and the duct 16 is formed by a parallel hollow pipe group 18. When a cooling fluid flows through the pipe 18, heat from a flue gas through the separator 14 from the reactor 12 passes through the pipe 18 and is spread to the cooling fluid, thus heating the cooling fluid. The cooling fluid is led to a lower header 26, then enters a pipe 28b there, passes through the separator 14, is passed to a container 32 via a pipe 31. As a result, the duct 16 reduces thermal loss and minimizes the request for the internal fireproof insulation. Heat is spread through the pipe 18 by the cooling fluid instead, thus increasing the efficiency of the steam- generating device.