This study attempts to reinterpret ambiguous boundaries based on post-structuralistic thinking that explored various meanings and possibilities beyond the binary confrontational relationship between boundaries and ambiguous boundaries, and to derive ambiguous boundary expression characteristics as a medium for indoor space expression of these thoughts. After establishing the concept of post-structuralism and ambiguous boundaries through theoretical considerations, we conduct a case analysis of art galleries that bring out creative thinking and provide rich experiences, targeting eight small art galleries in Gyeonggi-do, Korea, which opened in the 2010s. As a result of the analysis, post-structuralistic thinking that emphasizes various interpretations gives the viewer freedom to interpret and give meaning, and the ambiguous boundaries of space act as a medium in the expression of thought. The ambiguous boundary expression of the indoor space used at this time can be classified into five categories. First, it is the internalization of nature according to the use of natural materials, physical and psychological connection between the ceiling or curtain wall, inside and outside, and the second characteristic is the visual continuity such as the sense of depth of space, repetitive use of vertical and horizontal elements, and various interpretations of the central hall. The third characteristic is that the direct and indirect inflow of natural light enables a flexible experience depending on the passage of time and location, and the fourth characteristic is the organic spatial connection based on the use of multi-layering and horizontal and irregular elements, and the last is phenomenal ambiguity based on material properties such as transparency and reflectivity. Not only art museums but also many modern spaces require additional roles beyond their primary roles, so the expression of ambiguous boundaries that induce active participation and interpretation by the experiencer will become a very important concept.