The effects of culture density on the growth and fecal production of the oyster Crassostrea gigas in Hiroshima Bay were estimated using a numerical model expressing physiological processes of the oyster by changing the rate of water exchange into and out of the cultivation raft. While the individual-based growth of oyster was higher with decreasing culture density, total production was highest with 850 wires per raft. The individual-based fecal production increased with decreasing culture density, but the total amount of fecal production was highest with 950 wires per raft. From these results, it was concluded that 850 wires per raft was the appropriate cultivation density under the environmental conditions of Hiroshima Bay with no consideration of sediment deterioration.