We reinvestigated the composition and stratigraphical position of a plant macrofossil assemblage including Metasequoia glyptostroboides and Davidia involucrata, which has been reported from the upper part of the Sayama Formation, Kazusa Group in the Sayama Hills, west Kanto region. This fossil assemblage is composed of 24 taxa, including warm temperate plants such as Paliurus sp., Triadica sebifera var. pleistoceaca, and Vernicia sp., which indicate their deposition during an interglacial stage. Cool temperate taxa in the assemblage such as Picea sp. (sect. Picea) and Styrax obassia were possibly transported from mountains surrounding the sedimentary basin. We determined petrographic properties of tephra at the horizon immediately above the plant fossil layer. This shows that this tephra can be correlated to the Zoshiki tephra (ZSK) deposited in ca. 1.3 Ma. Stratigraphic occurrence of exotic taxa between the late Pliocene and Early Pleistocene in southwest Kanto region is similar with that in central Kinki region, which was influenced by warm and oceanic climate in this region facing the Pacific Ocean.Whereas, the ages of appearance of cold-loving plants as Picea maximowiczii in southwest Kanto region were earlier than those in Kinki region, which is possibly caused by distribution of mountains with high altitude areas where cold-loving plants have invaded much earlier.