This study examines the process by which the Yamato polity developed into the central government of ancient Japan, focusing on records found in the Nihon Shoki and Chinese official histories. By correlating diplomatic reports, internal reforms, and the structure of reign years based on the Spring-Autumn Double-Year Calendar System, the paper identifies the period between 239 and 266 CE as pivotal in the centralization of power. It also evaluates the historical implications of these records in light of previous studies and proposes a revised interpretation of early state formation in Japan. --- **Author's Note:** This paper is licensed under **CC BY-NC-SA 4.0**. (Uses outside the following conditions—such as for commercial purposes or without the ShareAlike provision—require prior permission.) - **Attribution (BY):** The author's name must be credited. - **NonCommercial (NC):** Free for non-commercial use, including summaries and translations. Commercial use requires prior permission. - **ShareAlike (SA):** Any derivative works (including summaries, adaptations, etc.) must be released under the same license (CC BY-NC-SA 4.0). **Contact:** For inquiries regarding this work (including permissions for commercial use), please visit the following website: 👉 https://tsurezureblog.sakuraweb.com/contact📧 h.shiina.contact@gmail.com Additions and revisions ・November 9, 2025