This paper aims to clarify the spatial composition of a mountain settlement through its formative process in modern times by the scope of the relationship between a water system and a village community in Ainokura, one of world heritage villages in Shirakawa-go and Gokayama. The following three points were clarified. 1. Three types in the relation between a water system and "kumi
", the smallest unit in a village community, are classified. Since they reflect on a classification as spatial units in a settlement, the space of a mountain settlement consists of them. 2. While each "kumi
" has a dominant family, there is a spatial framework and role in a settlement. 3. The framework of a settlement determines the direction of its expansion and contraction.