Si3N4 ceramics with Y2O3 and Al2O3 additives set in an Al2O3 sagger with a Si3N4 powder bed were successfully sintered in air atmosphere furnace at 1600-1700°C. Bulk densities of the sintered Si3N4 ceramics were 3.10-3.16g/cm3 which were similar to the densities of those sintered in a nitrogen gas furnace of 0.1MPa. No heterogeneous surface layer was observed. When the sagger and powder bed are structured so that SiO gas continuously generated during the sintering, Si3N4 ceramics can be sintered in air without serious oxidation.