The Southern Kanto "dissolved-in-water type" gas-producing region, which covers Chiba prefecture, a part of Tokyo, Kanagawa, Ibaraki and Saitama prefectures, is one of the largest gas-producing regions in Japan. The methane gas from the region has no geological relation to petroleum or coal. Stratigraphically most of the gas-producing horizons in the region are restricted to the marine Kazusa group of Pliocene to Pleistocene in age. (Here, the Kazusa group denotes all the formations between the Kurotaki unconformity and the top of the Kasamori formation.) The natural gas is concentrated in a basin-like structure of the Kazusa group, being mostly dissolved in connate brine water. It can be considered that the natural gas in the Kazusa group, at least the greater portion of it, is indigenous to the group. Roughly speaking, the natural gas potentiality is highest in the southern part of the Kujukuri district, and very low both in the western half of the area of distribution of the group and in the Kisarazu-Goi district. Judging from the geologic conditions during the deposition of the Kazusa group, such as lithofacies, depositional environments, and the modes of sedimentation of the group, the natural gas in the group was generated in the greatest quantities in the deep open-sea bottom where much argillaceous sediments were rapidly deposited. A considerable part of the gas thus generated has migrated into the porous layers in the group; however, since the time of the deposition of the group the gas has always behaved under the strong influence of the movements of the groundwater, that is, gravitative percolation of meteoric water into the strata, and compaction current in the strata. And, the movements of the groundwater have corresponded exactly to the changes in the pressure distribution of the water, which have been caused by sedimentation, deformation of the strata, and changes in paleogeography of the Kanto region ever since. Thus, the present modes of occurrence and the distribution of the natural gas in the Kazusa group can be clearly explained from the geologic conditions during the deposition of the group, and from the geohistorical factors which have more influentially controlled since the time of the deposition of the group than the former.