To develop a lumber dryer capable of reducing a cost of fuel expense, decreasing moisture content in lumber within a short time and preventing cytoclasis of lumber under utilization of geothermal heat and solar heat due to the presence of a heat insulation wall comprised of an outer wall part heat insulation material 22 of a building and an underground heat insulation material 1 added with water shielding properties that is connected to the underground side of the outer wall part heat insulation material 22 and inserted into the underground by more than 3 m.

This invention relates to a lumber dryer for a building utilizing geothermal heat and solar heat that is comprised of an under-floor heat radiation box 3 capable of sufficiently taking geothermal heat of 15 to 16°C at underground of no more than 3 m and performing natural dew condensation due to a temperature difference, and a lumber drying chamber 15 capable of taking solar heat of 40 to 60°C and adding the solar heat to the geothermal heat and performing earlier heating of lumber, and eliminates the need of fossil fuel, can decrease the water content rate of lumber, and prevents the occurrence of cytoclasis of lumber.