ＡＭＳによる放射性炭素年代測定法の高精度化によって，型式学を基礎として把握されたさまざまな考古学的現象の時間幅を精確に把握することができるようになった。それにより有効性が増した作業の一つに，土器型式ごとの遺構・遺物量の算定などからする人口変動の復元がある。本論では，弥生時代から古墳時代に向けてとくに顕著な社会の複雑化を見せた吉備中南部地域を対象に，これまでの発掘成果に基づき，当該時期の竪穴住居数・掘立柱建物その他の遺構数の変化を把握して，人口の増減や分布の変化過程をつかんだ。その結果，弥生時代から古墳出現期にかけてはこれまで想定されていた以上の人口の増加があること，とくに弥生時代中期後葉と後期とにその画期があることが判明した。さらに，その二つの画期にそれぞれ顕在化する考古事象の検討および古環境復元研究の成果などから，中期後葉の人口急増と集落数増加は環境の安定による人口増加率の上昇に起因し，後期の人口増加と特定地域への集住，そこでの階層化・耕地開発・遠距離交易活性化などの現象は，環境の不安定化に対応する社会的再編によるもので，それが古墳時代への移行の歴史的本質であることを展望した。Improved accuracy of the AMS (Accelerator Mass Spectrometry) radiocarbon dating method enabled researchers to precisely estimate the duration of archaeological phenomena apprehended based on the typology. One of the works whose validity was enhanced by the AMS technique is simulation of demographic changes based on the estimated amount of remnants of buildings and objects of each ceramic typology.This article reveals the change process of the number and distribution of the population in the southern central region of Kibi, where the society grew particularly complicated from the Yayoi period to the Kofun period. To this end, this research examined the changes of the number of dugout houses, horitate-pillared buildings (buildings with pillars embedded directly into the ground), and other construction remnants during the related period, based on previously excavated materials. The results discovered that the population from the Yayoi period to the beginning of the Kofun period, especially in the latter half of the Middle Yayoi period and in the Late Yayoi period, increased more than previously assumed.Moreover, paleoenvironmental reconstruction studies and further examinations on archaeological phenomena during the above-mentioned two periods of population growth revealed the reasons behind these increases. In the latter half of the Middle Yayoi period, the high population growth rate due to the environmental stability raised the number of population and settlements. In the Late Yayoi period, the population growth and its concentration in certain areas as well as phenomena such as the stratification of society, the cultivation of land, and the activation of long distance trade were attributed to social restructuring to deal with environmental destabilization. This article further develops the argument that the social restructuring was the historical essence of the shift toward the Kofun period.