A method for simply and efficiently separating substances by utilizing a specific flow behavior in a non-turbulent flow, i.e. a laminar flow, in a micro-channel is disclosed. A mixed solution containing at least two kinds of solute molecules which are different from each other in molecular weight and/or molecular sharp, or at least two kinds of solutions containing their respective solute molecules are flowed into a micro-channel to form a non-turbulent flow. A physical action is given to each molecule by changing the state of flow, thereby causing different behaviors among the different solute molecules. By utilizing this behavior difference, molecules of a specific kind are gathered in a specific region in the channel for separation.