Feedstock solution flow concentration system

A feedstock solution flow concentration method, comprising a first step for counterflowing or parallel flowing a feedstock solution flow containing at least a solute and a solvent and a draw solution flow via a forward osmosis membrane and transferring the solvent in the feedstock solution flow to the draw solution flow to obtain a concentrated feedstock solution flow, which is the feedstock solution flow which has been concentrated, and a diluted draw solution flow, which is the draw solution flow which has been diluted, wherein

• the feedstock solution is a pharmaceutical composition, and the number average molecular weight of the solute contained in the feedstock solution is 100 or more,
• the draw solution flow contains a draw substance and a solvent,
• the solvents of the feedstock solution flow and the draw solution flow both contain water,
• the feedstock solution flow and the draw solution flow each further contains a common solute,
• the common solute is a solute which is common between the feedstock solution flow and the draw solution flow,
• the concentration of the common solute in the draw solution flow is 1% to less than 100% of the concentration of the common solute in the feedstock solution flow, and
• the draw solution flow is prepared by dissolving or dispersing the common solute and the draw substance in the solvent of the draw solution flow before the first step.