PURPOSE: To separate efficiently monovalent metallic ions from a solution containing non-protein type nitrogen components by a crossflow method using a reverse osmosis membrane with a specified value for fractionating molecular weights.

CONSTITUTION: Monovalent metallic ions are separated from a solution containing non-protein type nitrogen components by a crossflow method using a reverse osmosis membrane with a specified value for fractionating molecular weights. A nitrogen cylinder 4 is connected with a circulating tank 2 through a pressure reducing valve 11 to replace the atmosphere in the circulating tank 2 with nitrogen. In a place membrane cell 1, the solution flows in parallel to the surface of the reverse osmosis membrane, and the filtrate is separated from a circulating passage through a hose 12 which hangs downward from the middle position of the cell 1. In this way, the desalting of the solution can be conducted economically by mass treatment.