The Northeast Japan arc is associated with westward subduction of the Pacific plate beneath the Eurasian plate. At the southermost region of the arc, the Philippine Sea plate subducts northward into the mantle wedge between the Eurasian plate and the Pacific slab. Akagi volcano located at this region yields volcanic rocks with combined Sr-O isotope relationships which are explained in terms of the mixing of the mantle material from the mantle wedge and deep ocean and/or continent derived sediments conveyed with the subducting Philippine Sea plate. This implys that the Philippine Sea plate continues to the location just under Akagi volcano, although the seismic Philippine Sea plate does not reach just under the volcano.