We found a high concentration of dissolved aluminum accompanied by almost neutral pH in stream water within a suburban secondary forest underlain by granite bedrock. We aimed to clarify the characteristics of the high Al levels in runoff by observing the chemical properties of spring water, stream water and surface soils. The mean pH values of spring water and stream water were 6.09 and 6.44, and the mean concentrations of dissolved Al of each after filtration with 0.45 μm membrane were 66 μmol/L and 41 μmol/L, respectively. The concentrations of dissolved Al in these waters were extremely high for such pH levels. There was no relationship between dissolved Al concentration and dissolved organic carbon (DOC), which, if it did exist, would be considered an indicator of organic species of Al in the stream water. The concentration of dissolved Al was highly correlated with Fe concentration but not with Si. The total concentration of water-soluble Al in soil extracts from the surface soils around the stream was extremely high, and the concentrations of dissolved Al, Fe and Si were highly correlated. These results suggest that the high concentration of Al in the spring water and stream water are derived from weathering of aluminosilicates and that Al might be dissolving in water as inorganic colloidal species.