To keep the temperature inside a drying chamber constant by composing an exhaust pipe of a plurality of parallel pipes and enclosing each of the pipes with granular refractory solid material.

Eight exhaust pipes (lower side exhaust pipes) 10a that go out from a combustion furnace 3 are extended approximately horizontally and connected to a connection box 11 arranged on the other side of the combustion furnace. Eight exhaust pipes (upper side exhaust pipes) 10b are extended from the connection box 11 approximately horizontally and connected to the lower part of a chimney 9. A long and narrow iron container 12 is formed surrounding the lower side eight exhaust pipes 10a and the container 12 is packed with pebbles 13. A fan 16 is attached to the upper part of a frame body 15 to agitate the air inside a drying chamber 1.