Provided is a method for processing a feed solution, which is an organic solution containing a small amount of water, using forward osmosis under a non-heated condition to obtain a dehydrated feed solution without causing degradation or change in quality of a solute. This method is for dehydrating a feed solution containing a first organic solvent, water, and a first solute, and comprises a dehydration step for bring the feed solution and an organic draw solution containing a second organic solvent into contact with each other through a forward osmosis membrane to obtain a dehydrated feed solution that has the moisture content thereof reduced to less than 1 mass % through dehydration. In one mode, the initial moisture content of the feed solution in the dehydration step is not less than 1 mass % but less than 30 mass %, and the initial moisture content of the organic draw solution is less than the initial moisture content of the feed solution.