A gradual population increase accompanying climate cooling has been evinced as having occurred in western japan during the Middle (ca. 5000-4000 years BP) to Late-Final Jomon period (ca. 4000-2300 years BP). We test the hypothesis that this population change paralleled increasing human migration. We also test the archaeological hypothesis that types of ritual tooth ablation can be used to distinguish between locals and immigrants during the Late-Final Jomon period. We measured strontium isotope ratios in human skeletal remains from the Middle Jomon Ota and the Late-Final Jomon Tsukumo sites located in the Sanyo region of western Japan. Tooth enamel and bone were analyzed, and modern plant samples were collected in the areas surrounding the two sites to make a map of environmental strontium isotope ratios. The biosphere strontium isotope ratios correlated well with the underlying geology, enabling us to put forth a hypothesis of immigrants' origins. There were no migration pattern differences between the Middle and Late-Final Jomon groups, indicating that the gradual population increase was caused by an increase in the indigenous population. All the Tsukumo individuals are locals, and this finding indicates that types of tooth ablation did not distinguish between locals and immigrants. Alternative hypotheses for the presence of different tooth ablation types in the Jomon society should be explored.