This study investigates the mechanism of semantic extension focusing on the dimensional adjective ‘usui’. ‘Usui’ is a dimensional adjective denoting a small physical thickness, but it has been extended to various meanings through metaphorical extension(metaphor and metonymy). In this paper, we divide ‘usui’ (thin) into a prototype meaning and five distinct meanings, and consider the mechanisms through which each of these meanings has undergone various extensions. Starting from the prototypical meaning of ‘usui,’ which refers to the state of one side of an object with parallel surfaces having a width smaller than the reference value, the meaning has extended to various domains. And from the prototypical meaning, it has derived the meaning① that expresses the state of low density or concentration of objects within a certain space, which has further diversified into various meanings. Particularly, ‘temporal adjacency’ has been a significant motivating factor for the semantic extension of ‘usui’. Temporal adjacency is one of the metonymic patterns based on the causal-process-result temporal relation, and it serves as a primary foundation when the prototypical meaning of ‘usui’ extends to a different sense.