The Tana-Mukaihara site is located on the left bank of the Sagami River in the Sagamino Upland in southern Kanto Plain (Honshu, Japan). In addition to the two lithic scatters (mainly composed of obsidian debitage and tools) and two pebble clusters, the site has been recognized as a prominent Upper Palaeolithic site (c. 20,000 cal.y.b.p .) because of an exceptional pebble-surrounded habitation area identified as a dwelling structure, 12 postholes, and two hearths. The purpose of this article is to examine human activities in this Palaeolithic dwelling site through the analysis of thermally altered obsidian artifacts and their spatial distributions. The results show that thermally altered obsidian artifacts are not only concentrated around the hearths, but also in the postholes, suggesting that the so-called postholes indeed functioned as hearths with the other two hearths. This study provides new insight into the reality of Palaeolithic dwelling structures in the Japanese Islands.