Recent enhancement of seismic networks in the Japan Islands revealed occurrence of low-frequency continuous tremors of a beltlike distribution in the southwest Japan, where the subducting Philippine Sea plate reaches depths of 30--40 km. Source depth of the tremor is estimated by selecting tremor segments with relatively clear P-wave onsets. The source region of the tremor is assumed to correspond to lowermost parts of crust close to a triple boundary of the crust, mantle wedge, and the subducting slab. The long duration of the phenomenon indicates existence of fluid relating to the generation of the tremor. The most probable fluid is considered to be water produced by dehydration process of chlorite and amphibole in the slab on the basis of data from high pressure and temperature experiments. The northern border of the beltlike distribution is possibly rimmed by the edge of the mantle wedge because serpentine formation absorbs fluid water.