The Sea of Japan is one of the back arc basins distributed around the western rim of the Pacific. Nature of the plume for back arc basin (= BAB) opening is unknown partly because petrological characteristics of the BAB mantle are not well known compared with mid ocean ridge (= MOR) mantle.Peridotite xenoliths in an alkali basalt dredged from the Takeshima seamount (tentative name), ca. 230 km to the north of Oki islands, are examined to clarify the petrological feature of the Japan Sea mantle. They are spinel bearing lherzolite to harzburgite with equigranular to porphyroclastic textures. They can be classified into two types (Type 1 and Type 2 peridotites) in terms of REE (= rare earth elements) patterns of clinopyroxene (= cpx). Type 1 peridotites (Cr# of spinels, 0.4 0.5) are similar to abyssal peridotites in terms of major element chemistry and middle to heavy REE concentrations of cpx. The cpx in the former shows slight enrichment of LREE (= light REE), which is apparently due to metasomatism by slab derived fluids/melts. Type 2 peridotites have high REE cpx with flat to slightly LREE enriched patterns, and are quite similar to subcontinental peridotites obtained as xenoliths from China. The southern part of the Sea of Japan was formed by mainly thinning and rifting of the continental lithosphere. Type 2 peridotites are a remnant of the continental lithospheric mantle.Chemical characteristics of BAB basalts and Type 1 peridotites indicate that open system melting of peridotite affected by a flux from the downgoing slab was involved in the Japan Sea opening.