A forward osmosis membrane flow system (1) is provided with: a high osmotic pressure fluid flow section (2) to which a high osmotic pressure fluid is supplied; a low osmotic pressure fluid flow section (3) to which a low osmotic pressure fluid having an osmotic pressure lower than the osmotic pressure of the high osmotic pressure fluid is supplied; and a semipermeable membrane (4) that separates the high osmotic pressure fluid flow section and the low osmotic pressure fluid flow section from each other. The flow rate in the high osmotic pressure fluid flow section (2) is increased by fluid migration from the low osmotic pressure fluid flow section (3) to the high osmotic pressure fluid flow section (2) through the semipermeable membrane (4). The semipermeable membrane (4) is a composite semipermeable membrane which is obtained by forming a polyamide-based skin layer on an epoxy resin porous membrane.