We examined the role of the spongy structure of the lower jawbone where an implant was placed. To evaluate the effect of the microstructure, the voxel FEM was used. The loading condition suited a biting motion, where the forces produced by six muscles were measured. The results of analysis showed that stress around the implant was widely distributed not only in the cortical bone but also in the cancellous bone. It was noted that the amplitude of the stress concentration in the cortical bone around the implant was extremely reduced, almost to one quarter, by the structure of the bone trabeculae.