A lumber prepared by artificially destroying pit membranes on cell membranes of cells constituting the lumber, and then easily attaining the dried condition of the lumber. In the lumber obtained from the wood cut down, a torus in a pit membrane portion closes one of, especially, a pair of pits or the opposite pit thereof, so that the water in the cell becomes hard to escape. In view of this inconvenience, the present invention aims at preventing the pit membrane portion from being closed, i.e., destroying such a pit membrane portion, whereby the escaping of the water from the cell is promoted after the destruction of the pit membrane portion. According to the present invention, the lumber to be processed is left as it is for a predetermined period of time in a treatment chamber filled with smoke due to a wood gas generated by a wood fuel burnt, whereby the accumulation of heat and multiplication of far infrared radiation are done by the far infrared radiation occurring when the wood fuel is burnt, components existing in the wood gas, and a far infrared radiation multiplication ceramic material provided in a combustion furnace or a highly dense lava accumulated as proper voids are made. A large amount of far infrared radiation is applied to the lumber to be processed, so as to prevent one pit or the opposite pit out of the two pits from being closed with the torus in the pit membrane in the lumber and generate hollow spaces in these pits.