The interface geometry at the subduction boundary plays one of the most important roles on evaluation of both seismic and tsunami impacts as the mega-thrust earthquakes occur between two plates. Although the general feature of the subducted slab at the mantle depth is clearly delineated from the seismicity in the Wadati-Benioff zone, the exact interface geometry at the crustal depth is hardly obtained from abundant earthquakes scattering in and around the interface between the two plates. Examination of seismic data recorded at the dense seismic array in the Tatun volcano group of northern Taiwan shows two unambiguous P-waves generated by shallow earthquakes offshore the Hualien area in eastern Taiwan. The detailed analyses of travel-times of both P-waves show that the 1^st P-wave was the direct wave propagating within the upper crust while the 2^nd P-waves were reflected from the subducted slab dipping northward ~23 degrees. This observation reveals the general interface geometry between the subducted Philippine Sea and the overlaid Eurasian plates at the crustal depth, where is often considered as the locked zone and accumulated a lot of seismic energy for release. Thus, the interface geometry obtained here provides important parameters for estimating the possible rupture plane along the mega-thrust fault between two plates in the westernmost Ryukyu subduction system in the future.