Melting phase relations at high pressures have been examined for four Mg-rich basalts from both the trench and the backarc sides of the Miocene NE Japan arc. All these basalts crystallize olivine and orthopyroxene on the liquidus at pressures lower and higher than the point of multiple saturation with those two phases, respectively. The pressure of multiple saturation is rather constant for all samples (1.0--1.4 GPa), suggesting that (1) there was no across-arc variation in the depth of magma segregation in the Miocene NE Japan arc, (2) the depth of magma segregation remains constant since 20 Ma in the trench side of the volcanic arc, whereas it has changed from 30--40 km to >60 km beneath the backarc side of the arc. This temporal variation may be caused by increasing thickness of the lithosphere beneath the backarc region associated with cooling of the mantle wedge after the opening of the Japan Sea backarc basin in the early Miocene. ¿ American Geophysical Union 1994