<p> Facing each other across the Kanmon Straits, the Japanese cities of Shimonoseki and Moji together demarcate the western entrance to the Seto Inland Sea. As a result of their geographical importance for traffic and commerce, both foreign and domestic, these cities initially developed as port towns. A significant point in their overall process of development was when the Meiji government recognized both Shimonoseki and Moji as locations of importance for national policy in terms of both economic and military matters. Given the growing focus on urban policy in both cities, a fast adoption of the city planning act of 1919 was followed by a subsequent period of successive city planning that continued up until the beginning of the Shōwa era (1926–1989). In this manner, both cities underwent the same process of modernization.</p><p> However, these cities differ in terms of the layout of their urban space. These differences are clearly visible in early city planning documents. It is possible to interpret the differences as a result of variations in the principles that guided urban planning in each city. However, another possible conclusion is that working with pre-existing urban spaces was prioritized over any planning principles. In order to investigate these interpretations, this project focused on relevant documentary literature, such as official documents. We have thereby clarified the nature of the first city planning for Shimonoseki and Moji, as well as the details of the city foundations prior to the drafting of the early plans. Previous research has not sufficiently explored these topics.</p><p> Through this research, we found that the city planning of both cities followed in the footsteps of pre-existing city foundations; for example, the ports that were constructed prior to the modern era. Of course, the intention behind the planning was important. In the case of city planning for Shimonoseki, for example, we see an inclusion of suburban areas within the planned zone, in order to mitigate urban sprawl. This decision was in accordance with modern urban planning theory. In contrast, the plan for Moji was larger scale plan in mind, where the neighboring urban regions would be included to form the North Kyūshū Industrial Zone. Nevertheless, in both cities, the zoning and street structure was often built upon the urban space established in the pre-modern era. It is difficult to conclude that either city was reconstructed as something genuinely new.</p><p> In sum, differences observed between the first city plans for Shimonoseki and Moji should be understood as results of their respective histories of development in the early modern era, rather than as variations in the principles that guided those plans. This is not to say that early city plans were not guided by certain principles. In the same period, there are other cases of excellent, principle-based city planning in Japan. Taking this into account, we can conclude that the manner in which each city initially developed as a port town influenced its subsequent city planning. We anticipate that in future research, this interaction between city planning and prior developmental history can be further clarified through comparisons with case studies of other port cities.</p>