PURPOSE: To detect easily a defect part of lumber by forming an unset resin layer over the surface of the lumber with a separating sheet between and performing heat press working, and finding chlorosis, etc., which is caused by the press working as the mark of a defect point.

CONSTITUTION: A resin sheet 3 impregnated with urea resin is mounted on the surface of plywood 1 with the separating sheet 2 between, and heat press working is carried out by a couple of upper and lower heat press plates 4 and 5 to set the resin sheet 3. In this case, if there is a defect such as a tunnel 6 and a recess 7 in the surface of the plywood 1, no pressure is applied to the part of the resin sheet 3 corresponding to the defect, so the unset resin does not flow to the part and the setting is deficient. Therefore, the part becomes a chlorosis part 8. Thus, the presence and position of the defect part is detected by this chlorosis part 8.