The light green mineral from the Oeyama nickel mine has been identified to be nickeliferous sepiolite being a very rare mineral by means of chemical analysis, electron micrographs, thermal curves and x-ray diffraction. Its occurrence seems to be the first recorded in Japan. The mineralogical properties generally agree with those of fibrous type sepiolites described in the literatures, but the thermal effects on this mineral are somewhat different from those on the fibrous type. From the electron micrographs and X-ray analyses, it has been confirmed that the Oeyama nickeliferous sepiolite is composed of fibrous particles which are not uniform in length and have higher crystallinity than that of the sepiolite from this mine described recently. The chemical formula for this mineral was 12 SiO₂•8 (Ni, Fe, Mg) O•17H ₂O. Of this 17H ₂O obtained from the chemical analysis 3H ₂O were considered to be possibly hygroscopic water in comparison with the Preisinger formula.