Analysing the structure of rural regions as well as nodal regions formed by central places is indispensable in order to comprehend the regional structure in the Edo era. From the viewpoint of spacial structure, this article chiefly aims to analyse the regional organizations among the rural communities in terms of irrigation. Firstly, with due regard to the process of cultivation and the development of irrigation systems in Tonami District of Toyama Prefecture, the author divided the area into eight regions Region A1, A2, A3, B1, B2, B3, C and D -in compliance with water systems. Secondly, the author examined the regional differences and the interrelations of various factors, i.e., natural conditions, the irrigation policy of the Kaga-han (feudal clan), the scales and the management systems of Yosui (irrigation associations), which were regarded to be related with the irrigation systems. The summarization of the study or each region is as follows Region Al on the Shogawa Alluvial Fan had abundant water supply from the Sho-gawa River and was cultivated rapidly at the beginning of the Edo era on account of the political stability, the flood prevention works and the improvement of irrigation systems. The scale of each Yosui was large and had a tendency to be enlarged. Sub-units of Yosui among communities were formed accordingly, and two different levels of managing Yosui possibly developed as a whole. After all, the irrigation system was open in spacial struc-ture and, at the same time, loose in the connection of communities. Further, there were found two other types of irrigation system ; Goko Yosui and Heigo Yosui. In Goko Yosui, a union of Yosui, they cooperated to draw water from the river, and yet each Yosui kept its individuality. In Heigo Yosui, a dominate Yosui annexed smaller or less powerful ones, and the weaker Yosui gradually lost its individuality being finally reduced to a sub-unit. Region B1 and A2 were similar to Region Al to some extent. Region C which was irrigated by small rivers or fountains had been cultivated early in the Edo era and maintained its own irrigation system. Since it suffered from water shortage, the irrigation system remained unimproved. The scales of Yosui were small and their areas had not been enlarged. After all, the irrigation system was close in spacial structure and, at the same time, tight in the connection of communities under the strict, traditional regulations of irrigation and cultivation. Both Region A3 and B3 were similar to Region C. Region B2 and D can be placed between Region Al and C. Therefore, the spacial structure, the management system and the water right in the irrigation system show remarkable regional differences which were caused by the scale of Yosui, the irrigation policy of Kaga-han and the degree of sufficiency of water supply. Thus, the regional differences correspond closely to the abovementioned regional divisions.