Distinct reflected waves (S x S) are observed in the northern Kinki district, southwestern Japan. We conducted a high-resolution reflection analysis by using data from 128 seismic stations with an average spacing of about 5 km. We used a stacking method to obtain three-dimensional distributions of relative reflection strengths of S waves and found a thin planar zone of high reflection strengths at depths of 25-30 km, which we call a S wave reflector. We also found that the zone of high reflection strengths is dipping to the north and that low-frequency earthquakes (LFEs) occurred near the edge of the zone at depths around the Moho discontinuity. It is inferred from these results that fluid is concentrated in this zone of high reflection strengths. It is likely that the zone of high reflection strengths is a path of fluid upwelling from the mantle, together with the hypocentral region of LFEs, that is located near the lower edge of the zone. The northern Kinki district is thought to be part of the Niigata-Kobe Tectonic Zone (NKTZ) high strain rates region. The high reflection strengths zone may be associated with high strain rates in the NKTZ.