This paper traces the trajectory of North Korea’s historiography on ancient history, examining the political and social contexts surrounding the excavation (1974) and renovation (1993) of the Tomb of King Tongmyŏng. The 1974 excavation was a political project aimed at deepening the understanding of ancient history centered on Koguryŏ, an ancient kingdom whose capital was in P’yŏngyang along the Daedong River. The 1972 constitution, which established P’yŏngyang as the capital and proclaimed the Juche ideology, was a driving force behind this project. Over time, the P’yŏngyang-centric viewpoint on ancient history grew stronger, culminating in the excavation and renovation projects in the early 1990s of the tombs of Tan’gun, Tongmyŏng, and Wang Kŏn. The assertion that the legitimacy of Korean history is rooted in P’yŏngyang was used to justify North Korea’s own legitimacy, a concept this paper refers to as the "Principle of P’yŏngyang Legitimacy." In this context, the projects involving the tombs of Tan’gun and Wang Kŏn should be understood as extensions of the renovation of the Tomb of King Tongmyŏng, which had been underway since the late 1980s. In other words, the series of excavations and renovations of the tombs of the founding kings in the early 1990s originated from the Tomb of King Tongmyŏng, which, in turn, inspired the emergence of the "Principle of P’yŏngyang Legitimacy." This legitimacy is constructed as a linear genealogy belonging to a single subject, which is endowed with a legitimate self-identity, while entities outside this narrative are systematically excluded. The evolution and application of the "Principle of P’yŏngyang Legitimacy" serve as a caution against the politicization of history in South Korea’s historical research programs.