An analcime concretion was found in mudstone of Miocene Koori Formation in Dogo Oki Islands, Japan. The Miocene Koori Formation was mainly formed in a non-marine depositional environment. The analcime concretion consists of distinctly separated outer crust and inner core. The soft inner core is mainly composed of kaolinite, well crystallized metahalloysite (halloysite 7Å), and montmorillonite. While analcime with some detrital quartz and authigenic pyrite are major constituents of hard outer crust. No carbonate minerals are present in the analcime concretion. Porosity of the inner core was evaluated as about 50 to 60％, suggesting that this concretion was formed in the very early stage of diagenesis. Kaolin minerals characterized by low Si/Al in the inner core indicate that the inner core was formed in a relatively acidic and fresh pore water. The abundant analcime and pyrite in the outer crust show that the outer crust was formed in a Na^+ and SO_4^(2-) rich saline pore water. Analcime, however, can not be considered to be the primary mineral, since its formation temperature is generally more that 80℃. Clinoptilolite can be a candidate as a possible primary mineral formed from saline pore water rich in dissolved silica derived from felsic pyroclastic rocks which are commonly observed in this stratigraphic horizon. No evidence of hydrothermal alteration is observed. The occurrence of analcime concretion is possibly concemed with that the marine water invaded the fresh water depositional environment in late Koori Formation deposition.