This paper analyzes the achievements of ancient Japanese rulers, focusing on three key aspects that were emphasized during their later years: divination, succession planning, and the establishment of governance systems. In ancient Japan, rulers used divination to legitimize political decisions and establish the sacred authority of the monarchy. Furthermore, the selection of successors was seen as a crucial process for preventing succession crises and ensuring the continuity and legitimacy of the monarchy. Rulers in their later years carefully selected their heirs based on their abilities and qualities, ensuring that they gained experience in governance in order to facilitate a smooth transfer of power. Lastly, as these rulers entered their later years, they focused on the establishment of governance systems, restructuring the national administration, aspects of society, and key economic structures, all while strengthening their centralized control. These efforts played a key role in securing their legitimacy, the sustainability of the monarchy, and acting as a strategic measure to reinforce political authority. Typically, in their earlier years, Japanese rulers focused on expanding their influence through regional inspections and military campaigns, however in their later years, they sought to solidify their political position through institutional reforms, thereby consolidating their governance, authority and legacy. This establishment of governance systems (in their later years) can be interpreted as an essential process for enhancing political authority and ensuring sustainable governance. Throughout the analysis of these ancient Japanese rulers' achievements, this study sheds new light on their missions and social roles in their later years of rule.