The weathering process of Mannen altered andesite in Tobe Town, Ehime Prefecture, Japan, has been investigated based on geology, petrography and geochemistry. The Mannen andesite intrudes into the Sanbagawa metamorphic rocks and the Middle Miocene Kuma group. The andesite dyke have undergone extensive hydrothermal alteration and mineralization, and has domains of a high concentration of As, S and Sb. Index alteration minerals are pyrite, arsenopyrite, marcasite, stibnite, quartz, carbonate, chlorite, titanite and illite. The weathering types in the near surface of the Mannen andesite is divided into five based on the degree of progress of weathering. That results show that domains of sulphide minerals-rich altered andesite are extremely weathered. CaO, Fe_2O_3, MgO, Pb, Cu, Sb, S and As are depleted in weathered parts of the Mannen andesite based on isocon analysis. Especially, Fe_2O_3, S and As are extremely depleted in weathered parts of sulphide minerals-rich altered andesites. It shows that sulphide minerals in the andesite are decomposed by interaction of surface water and microbial activity, and these elements are leaching away to surrounding environments.