In 1910, Japan annexed Korea and integrated it into the Empire of Japan. According to its policy of assimilating colonies, the Japanese government intended to remove the tariffs between Japan and Korea, an aim which had almost been realized by 1923. The removal of the tariff barrier was supposed to improve market access between Japan and Korea. This article explores the implications of this event, focusing on the spatial distribution of economic activity in Japan. The regression results suggest that the integration of the Korean market increased population growth rates more in the regions close to the former border between Japan and Korea than in the other regions. Furthermore, after integration, the regions close to Korea that specialized in the fabric industry, whose products were the primary goods exported from Japan to Korea, experienced more population growth than other regions close to Korea did. These results suggest that market accessibility was indeed a determinant of the spatial distribution of economic activity. Our findings also indicate that the economic effect of colonization on the mainland was spatially heterogeneous and that a spatial viewpoint of the history of imperialism is important.