To increase the fastness/hardness and the strength of a bamboo quality as a bamboo material and minimize the stretch and shrinkage of the bamboo by setting the thickness, the number of splittings, thickness or the like of a material bamboo within a specified range respectively and employing a carbonization treatment by a carbonization device and a drying process thereafter.

A material bamboo with 80-120 mm thickness is split into 8-10 pieces in the sectional peripheral direction, and these 8-10 pieces are cut off and further, their moisture content is set to be about 30% or less by solar drying or the like. After that, the bamboo pieces are finish-molded using a roller molder. In the following step, the bamboo pieces finish-molded to the 20-32 mm width and the 7-12 mm thickness, are carbonized using a carbonization device 6, and are loaded into a hot blast dryer to be dried by hot blast. In the carbonization device 6, the process to reduce a pressure to draw a vacuum and to apply a water vapor and the pressure is repeated. The carbonization device 6 increases the pressure of a saturated water vapor and thereby unifies the moisture content of the bamboo and achieves the uniform rise of the temperature of the moisture content. Consequently, the uniform boiling of the moisture content and the averaged dry state of the bamboo material are realized by blowing off a saturated water vapor in the device 6.