Two different plates are subductiog beneath the Japanese island arcs, the Pacific plate in the north and the Philippine Sea plate in the south. Geophysical evidence indicates that the Philippine Sea plate subducts without extending beneath many of the volcanoes in the southwest Japan arc. The N2/Ar ratio (424) of a common magmatic gas from geothermal and volcanic areas in southwestern Japan is an order of magnitude lower than the value (4250) in northeastern Japan. Because the high N2/Ar values reflect input from subducted sediments, the large difference in the N2/Ar ratios of magmatic gases indicates that the contribution of subducted sediment to the source of magma in southwestern Japan is much lower than that in northeastern Japan. The N2/Ar ratios off magmatic gases from island arcs elsewhere may also yield information regarding the degree of contribution of subducted sediments to arc magma sources and differences in the states of plate subduction.