The Kofu Basin is a tectonic basin in the South Fossa Magna. On the southeast and west margins of the Basin, there develop two tectonically peculiar terrains the Sane Hills and the Ichinose Upland bordered with active faults. These boundary faults are considered to play an important role in the tectonics of the Basin, but those subsurface structures are not yet known well. We carried out a dense gravity survey in and around the Basin and contoured a detailed Bouguer anomaly map in the area concerned. On the west and southeast of the Basin, steep horizontal gravity gradient zones distribute in parallel along the active faults and they develop on the mountain side of the faults. This gravity feature suggests that the mountains are thrust over the Basin. We made two-dimensional subsurface structure analysis along two sections to cross the Sone Hills and the Ichinose Upland. It is concluded that the density boundaries are on the northern edge of the Sone Hills and on the western edge of the Ichinose Upland, respectively.