We have investigated gravity anomalies around the Niigata plain, which is a sedimentary basin in central Japan bounded by mountains, to examine the continuity of subsurface fault structures of a large fault zone-the eastern boundary fault zone of the Niigata plain (EBFZNP). The features of the Bouguer anomaly and its first horizontal and vertical derivatives clearly illustrate the EBFZNP. The steep first horizontal derivative and the zero isoline of the vertical derivative are clearly recognized along the entire EBFZNP over an area that shows no surface topographic features of an active fault. Two-dimensional density structure analyses also confirm a relationship between the two first derivatives and the subsurface fault structure. Therefore, we conclude that the length of the EBFZNP as an active fault extends to ~56 km, which is longer than previously estimated. This length leads to an estimation of a moment magnitude of 7.4 of an expected earthquake from the EBFZNP.