The resistance of heat-modified sugi (Cryptomeria japonica D.) sapwood against mold and decay fungi was evaluated under laboratory conditions. Wood specimens were subjected to heat treatment at 180°C for 2 and 4 hours and at 220°C for 2 hours. Changes in lignin and polysaccharide weight percent composition, solubility, and pH of heat-modified specimens were determined. Treatment at 220°C for 2 hours decreased wood polysaccharide content. Heat treatment slightly increased resistance against Rhizopus javanicus and Gliocladium (Trichoderma) virens, but not against Aspergillus niger. The highest temperature used increased resistance to decay by the white-rot fungus Trametes versicolor, but mass losses with the brown-rot fungus Fomitopsis palustris were higher than those with T. versicolor.