The Early to Middle Miocene deposits are distributed in the Chichibu basin in the northeast of the Kanto Mountains. The Yokoze-macni Group which intercalates some thick conglomerate beds, occupies the uppermost part of the Miocene deposits. The characteristics of the conglomerate beds and their sedimentation process are as follows: (1) Five conglomerate bodies have been recognized in the Yokoze-machi Group. They are Lower Sekinoiriya, Upper Sekinoiriya, Nakago, Une and Karugome conglomerate bodies respectively, in ascending order. (2) The four conglomerate bodies, Lower and Upper Sekinoiriya, Nakago, and Karugome, are composed mainly of angular clasts of sandstone and slate derived from the basement rocks. These conglomerate bodies tend to be thicker toward the east, suggesting that much of detritus was supplied from the basement rocks wich distribute to the east of the basin and is presently in fault contact with the basin-fill. (3) The Une conglomerate body is composed mainly of angular gravels of sandstone and mudstone derived from the Neogene deposits. These angular gravels were probably supplied from southern (southwestern) or northern (northwestern) areas in the basin. (4) The conglomerates containing many angular gravels derived from basement rocks are supposed to have been deposited from subaqueous debris flows or other high-energy flows in the shallow-water embayment, 10 to 100m deep.