PURPOSE: To make young wood, that is, wood which is moistened but contains a large amount of combustible used in a hearth bed, by introducing a thermal gas extracted from combustible to pipes disposed at the underside of a bottom so as to heat the bottom to a temperature which enables an evaporation of a gas component.

CONSTITUTION: A bottom portion 7 is made of a material which is hardly dispersed or dissolved, steel or ceramic and has a planar shape which inclines in a frontward direction. The bottom portion 7 is disposed upstream of the pipe 10 which works as a thermal condenser. The evaporation of liquid combustible is substantially carried out at the upstream side of the pipe 10 and the dispersion of heat quantity necessary for the evaporation is generated at the downstream side of the same pipe 10. Gas and smoke are introduced into a space 11 positioned beneath the bottom portion 7 and the gas makes the bottom portion 7 the same temperature as the thermal gas temperature. Since the temperature of the bottom portion has not yet reached a condition which induces heating of combustible wood, in the initial operation, an ignition wood or a dry wood is used. Starting from the space 11, the gas circulates toward a vertical peripheral space 4 and reaches a peripheral horizontal space 5 which is located above a fire box 3 and thereafter reaches the upstream of an exit 2.