The incomplete treatment of the swinery wastewater is one of the major factors of the river contamination resulting from an eutrophication. Even through many treatments were considered to reduce the contamination of the river by the swinery wastewater, the most effective treatment was not developed yet. Therefore, this project was focused on the development of the treatment that was a low cost and a high efficiency using a steel-making slag. The swinery wastewater was passed through a U-type tube packing the slag in the laboratory. And the swinery wastewater was cleaned effectively in the laboratory experiment. Based on these laboratory results, the treatment facility in a stock farm was constructed to confirm the effect of slag on the first-treated swinery wastewater. After the treatment of the first-treated swinery wastewater through the slag, the water quality of the river was improved and the biodiversity was increased.