The invention provides an apparatus for treating a river water, comprising a pit type bath built below a river bed on the path of flow, the pit type bath being wider than half of an effective flow width of the river, being more than 2 meters deep and having length of more than 10 minutes retention time, and aeration means equipped in the pit type bath for aerating and whirling the water which flows into the pit type bath. The said device does not reqire any work mechanism except aeration means equipped in the pit type bath and pipe for providing pressured air. Besides the blower for generating pressured air, the invention does not consume any utility cost and medicine cost, and the blower is used to generate swirl and the sand does not accumulate at the bottom, and the accumulation of rock can be removed at fixed periods and the maintain cost is low.