Streams transport various nutrients such as NO₃^- and PO₄^3- to the downstream. These nutrients are loaded by precipitation and leachate from forest soils, and move into the lake and the sea. Also in stream, these nutrients are produced by mineralization, and are taken up. Such nutrient uptake and emission are important determining factors of nutrient supply to the lake and the sea. Since 1980’s, nutrient addition experiment, which directly measures NO₃^- and PO₄^3- uptake in streams has been widely applied. In some forest streams of Japan, results of such experiments showed that PO₄^3- uptake was rather smaller than those of streams in United Sates, and NO₃^- uptake was rarely observed. These suggested that some factors might restrict the nutrient uptake in Japanese stream. However, it is generally difficult to determine those in-situ conditions of the actual stream, because nutrient uptake is usually made by multiple communities of stream algae, fungi, and bacteria. Their activities are strongly influenced by a number of surrounding conditions such as lights, biomass, and so on. We used a stream simulator to investigate the restrict factor of nutrient uptake in a forest stream in Japan. Stream water and biofilms, which has been breeded in-situ condition of the stream, have been used. We observed GPP and nutrient uptake in various conditions.