We estimated thermal structure in the mantle wedge of NE Japan by applying the experimental results of olivine-dominated rocks to seismic attenuation data. Obtained temperatures at a 40 km depth are 1000--1130¿C and 960--1090¿C beneath the volcanic front and the back-arc side, respectively. We cannot obtain the areas with a temperature higher than 1400¿C in the mantle wedge, which have been inferred from petrological studies. This is perhaps due to the localization of such anomalous areas. The obtained temperature is higher than that of the wet solidus of peridotite in the greater part of the back-arc side, which suggests that the addition of aqueous fluids plays an important role in generating melts in the mantle wedge.