Smallpox is a contagious, airborne, viral disease that infects only living creatures. After recovery from the disease, they acquire lifelong immunity. In Japan, smallpox was already known in the Nara period, many of the infected were children, and it caused cyclical outbreaks in various areas. However, little is known about the effects smallpox outbreaks had on local communities and population structures or how it was transmitted. This is because methods to prevent smallpox such as mass variolation and bovine vaccination (smallpox vaccination) were established and widely used early in history. Thus, much remains unknown globally about outbreaks among people without immunity.<br>The author found a record of infections in a region where a smallpox outbreak occurred during 1795–1796: Nakatsugawa, Dewa province, northeast Japan. Nakatsugawa consisted of 14 villages, and a document exists recording the date of the onset of the smallpox outbreak, number of people affected, number of deaths, and number of those susceptible in each village. This article examines the spatial process of smallpox diffusion in the rural district of Nakatsugawa during that 1795–1796 outbreak. The majority of the infected were children under the age of 10 years, and this pattern was common among smallpox epidemics in early modern Japan.<br>The smallpox outbreak began on August 7, 1795, in Kamiyachi, a village located at the western end of the district, through which the main road to the adjacent province passed, and spread slowly to neighboring villages, most of which were composed of several small settlements. The time lag of the onset of smallpox between Kamiyachi and the other villages showed a strong correlation with the distance by road between them, with a correlation coefficient of r=0.74. However, children's mobility was so low that it took an average of one month to transmit the disease between adjacent villages. It is presumed that the range of movement of children aged 10 years or younger was less than 2–3 km. During this period, some children aged between 8 and 10 years attended private schools, which were typically located less than 3 km from their homes. Weather conditions such as snow in winter slowed the diffusion rate considerably. Moreover, detailed analysis of the patterns of outbreaks in some villages revealed that smallpox was transmitted by contact between playmate groups of children formed in every settlement. Symptoms of smallpox appeared almost simultaneously among children of various ages in the settlements concerned, and among siblings in the same household. Thus, among those susceptible more than 86% in this district became infected. This point is important when analyzing the reincrease in the number of susceptible individuals in rural areas.