Serpentine is one of the candidates to explain low-velocity and high Vp/Vs anomaly in subduction zones. However, the extremely high Vp/Vs found beneath Kanto and southwest Japan suggests the presence of aqueous fluids in addition to the serpentinized mantle. In this study, we investigated the effect of pore fluid pressure on elastic wave velocity of antigorite serpentinites at confining pressure (Pc) of 10-200 MPa, pore pressure (Pp) of 10-100 MPa and room temperature. Experimental results under dry conditions show that compressional and shear-wave velocities increase with increasing confining pressure, and that Vp/Vs increases slightly with pressure. Under wet conditions, the effect of pore fluid pressure depends on the sample porosity. In the sample with relatively high porosity (0.8%), elastic wave velocities are sensitive to pore pressure and are mostly unchanged at a constant differential pressure, Pc-Pp, within the studied range of confining pressure. In contrast, the sample with low porosity shows a little influence of pore fluid pressure, in which the elastic velocities are insensitive to differential pressure, but are sensitive to confining pressure. In the former sample, Vp/Vs increases with increasing pore pressure at a given confining pressure; however, the change in Vp/Vs is relatively small, suggesting that the observed extremely high Vp/Vs requires the infiltration of large amounts of water probably due to a hydro-fracturing at the plate interface.