A grain conveyor structure was developed to reduce the time required for internally cleaning a combine harvester. In the developed structure, the horizontal plane parts in the grain conveyor were slanted to promote slipping of grains. Further, the cleaning ports were enlarged and located in optimized positions to enable the easy discharge of grain residues. The results of the examinations conducted using a head-feeding combine harvester indicated that the time required for the removal of grain from the grain conveyor of the developed structure was approximately half of that required using the conventional structure.