Recently, problems caused by seaweeds have been recognized in Japanese society, whose land is surrounded by the sea. Seaweeds glow in large quantities and pile up on the seashore (the green tide). 2 Figure 1 shows an example of such green tide. They spoil the view and release odor as they rot quickly. Ulva sp. is the most typical among those seaweeds. In Japan it is often marketed as a material for food. However, such frequently encountered Ulva sp. is not of the edible quality. After having been collected by local governments, a massive amount of seaweeds are being incinerated. Recently, seaweeds have been employed for ocean remediation, instead of being consumed as food. It is expected that they may protect fishery fields from waves and reduce nutrient in the sea. Cultivating Laminaria sp. has been attempted for such a purpose. Despite its effectiveness, the treatment of harvested seaweeds has arisen as a new problem. In Japan, such unwanted seaweeds are abundant and, therefore, effective utilization of them is highly desirable. Extracting energy from them is one such example. It contributes as a means of ocean remediation and can serve as a remedy against the green house effect. In the existing literature on the methane fermentation of seaweeds, merely results of laboratory scale experiments 3, 4 have been reported. In the present study, a field test plant was built for the large scale practical processing of biogas from seaweeds. The maximum treating capacity of the plant is one ton-seaweed/day. The present plant is equipped with a gas engine power generator. At the same time, an effective method of converting biogas into energy has also been pursued. The present field test is a collaborative project with the New Energy and Industrial Technology Development Organization (NEDO) of Japan.