Katata estate in the province of Omi is well known as having been a mikuriya, a specially designated estate to provide resources, of Kamo Shrine, but there has been debate about how to assess its fishing in the early medieval period. In previous scholarship there were some who took the position that saw it as a community with special privileges to fish freely in Lake Biwa, but an opposing view held that such privileges had not been established in the early medieval period and that the Katata fishermen constituted instead a rather weak group. Given the limited number of historical sources, in order to understand the reality of fishing during the medieval period, we must deepen our interpretation of written sources in light of natural conditions. Regarding the natural conditions of Lake Biwa in particular, there is one aspect unlike those of any other body of water in Japan: because it is an ancient lake with a history of over 4 million years, there are endemic species that evolved uniquely there and that were important objects of fishing. In addition, as it has a depth that reaches 100 meters as a result of it having been created by several geological faults, the various features of the lake bottom became important locations for endemic livelihoods. Having considered the geological features of Lake Biwa and the ecological behavior of varieties fish inhabiting the lake in this fashion, I attempt in this paper to reproduce the technological stages of fishing in each age and make clear the position of Katata estate in terms of medieval fishing. In this paper I use the fishing disputes between medieval Katata and the estates of Otowa and Sugaura as objects of analysis, and reproduce the fishing technology possessed the Katata fishermen from the types of fish caught and the fishing season. As a result, I realized the key to understanding the fishing at Katata in medieval times was the fishing technology that could operate in the deep-water fishing grounds. Among the fishing technologies that were possessed exclusively by the fishermen of Katata were gill nets (sashi ami) designed for the funa that were unique to Lake Biwa and longline fishing (haenawa ryo) used for other fish such as koi and biwamasu. Among these, it can be confirmed that gill nets were used from the Kamakura period onward, and I have pointed out that longline fishing was employed in the northern area of Biwako during the medieval period. In other words, I have made clear that in contrast to ordinary shoreline villages that only had fishing methods for shallow waters up to 15 meters deep, it was possible for the fishermen of Katata alone to fish in deep waters. The fact that at Lake Biwa not only the coastal zone, but the deep-water offshore zones were ecological spaces for varieties of fish held great significance. It has become clear that the fishermen of Katata held a monopoly on waters up to 60 meters deep in the vast off shore areas of Lake Biwa as their fishing grounds from the early medieval period onward through the use of the fishing technologies of gill nets and longlines. In these waters Katata faced no disputes with other groups and could practice fishing exclusively, which leads to the conclusion that Katata's superiority continued from the early medieval period onward. It can be recognized there was a degree of deference on the part of Katata to the fishing grounds of kugonin, who provided tribute to the court, such as the estates of Sugaura and Awazu, and there are signs of their exclusion from waters nearby these estates. Nevertheless, because the fishing methods shared by the kugonin of Sugaura and Awazu did not go beyond technologies for shallow-water fishing, there was no possibility of their fishing in the extensive fishing grounds in the deep waters in the offshore zone of Lake Biwa. It can be hypothesized that this was due in part to the fact that Katata alone continued to hold preeminent power in the Lake Biwa fishing industry until the early-