Abstract: The purpose of this study is to reduce the back clearance and roughness during longitudinally slicing of lamella by heating knife. It was originally studied in the paper that thick veneer would be longitudinally sliced through automatically heating knife with temperature controlling system of hot oil. The knife temperature was monitored with infrared thermograph. The back clearance and roughness of veneer were measured with heating and unheated knife. It was clarified that the vapor pressure was produced in cutting layer, and the material the back clearance rate of the material was decreased by 20 %, and the roughness of veneer was improved by 15 %—25 % when the knife was heated.