This paper examines the impact of South Korea's rapidly declining population on its defense capabilities and suggests policies to design military manpower system for sustainable defense. The rapid population decline is adversely affecting the entire military manpower system of R.O.K. based on the conscription system. As a result, we are focusing on solving the shortage of military resources from the quantitative perspective, including the supply and demand of military resources. In addition to that, I believe that in order to increase combat capability, we must simultaneously improve the quality of defense personnel. To meet this end, I propose a through review of the demand for military personnel, total defense manpower management system, introduction of a contractual soldier system, and strengthening the job security of professional soldiers as major policy designs.