Detailed petrological study was made on the felsic volcanic rocks dredged from the Meiyo-Daini seamount in the Yamato backarc basin, Japan Sea. The Meiyo-Daini seamount was created as a subaqueous volcanic edifice together with the other seamounts in the basin at the latest stage of backarc rifting. The issued felsic volcanic rocks contain mafic inclusions with varying sizes and textures. Mass balance calculation suggests that the host felsic magmas were differentiated from the mafic parent by fractionation of amphibole and plagioclase feldspar. On the other hand, the derivative felsic magmas have phenocryst assemblages which are different from that expected from the fractional crystallization model. These puzzling phenocryst assemblages are probable reflection of the following magma plumbing system; the felsic magmas were produced in deep-seated magma reservoir(s) (＞2 kbar;＞ca. 7 km below surface), and then ascent to crystallize the present phenocryst assemblages consuming the crystals relevant to the differentiation. The mafic inclusions could be less differentiated portion of the deep-seated magma reservoir(s). The inferred magma plumbing system is consistent with the fact that the Yamato backarc basin is underlain by anomalously deep seismic Moho.