Seismic tomography shows that slow velocity anomalies exist in the mantle wedge and extend to the forearc region down to the subducting Philippine Sea slab under northern Kyushu. We also conducted numerical simulations with petrologic data to estimate the fluid distribution in the mantle wedge. The seismic and simulation results indicate that regimes of melting and magmatism in a subduction zone with a young slab are different from those with old slabs. Dehydration and melting occur beneath the arc and forearc above a young (and warm) slab such as in northern Kyushu (≤26 Ma), while fluids (aqueous solution and melt) occur mainly beneath the back arc above old slabs.