Detailed seismic velocity structure in and around the source region of the Geiyo earthquake within the Philippine Sea plate was obtained using the double-difference tomography method. The obtained velocity structure and its comparison with the source fault of the Geiyo earthquake yield the following features. (1) The mantle of the subducted Philippine Sea plate is imaged as a high velocity structure, above which the low velocity anomalies are distributed. These low velocity anomalies probably reflect the crust of the subducted slab, within which most of intraslab earthquakes occur. High V_p/V_s values for the crustal part of the slab suggest that these intraslab events occur due to the dehydration embrittlement. (2) Source areas of the deep low-frequency seismic tremors are located in low velocity and high V_p/V_s ratio regions at depths of 30-35km (western Shikoku), which suggests that they are caused by fluids generated by various dehydration reactions inside the slab. (3) The hypocenter of the Geiyo earthquake is located near the Moho of the subducted slab, and coseismic rupture area and relocated aftershocks of the Geiyo earthquake are distributed within both the crust and mantle of the slab. Later phase analyses for aftershocks support the above observations; the coseismic slip extended not only into the slab crust but also into the slab mantle. (4) Large coseismic slip areas are distributed in relatively high velocity areas. (5) Low velocity and low V_p/V_s ratio zone is distributed in and around the source region of the Geiyo earthquake, which is different from velocity structures outside the Geiyo source region. This suggests that minerals including fluids such as CO₂ are distributed unevenly in space.