A method for manufacturing a forward osmosis composite membrane module having hollow fiber bundles composed of a plurality of hollow fibers; wherein the hollow fibers are hollow fibers provided with a separation active layer of a polymer thin film on the inner surface of a microporous hollow fiber supporting membrane and wherein the polymer is a polycondensation product of: at least one or more types of a first monomer selected from polyfunctional amines, and at least one or more types of a second monomer selected from the group consisting of polyfunctional acid halides and polyfunctional isocyanates, the membrane area of the hollow fiber bundles is 1 m 2 or more, and the coefficient of variation of average thickness of the separation active layer in the radial direction and lengthwise direction of the hollow fiber bundles, as calculated according to a method as described in more detail herein consisting of measuring the mass of the separation active layer portion in a scanning electron microscope image of a cross-section of the separation active layer in the direction of thickness, is 0% to 60%; the method comprising going through the steps of: forming a liquid film of a first solution containing either the first monomer or the second monomer on the inner surface of a microporous hollow fiber supporting membrane, providing a pressure difference between the inside and outside of the microporous hollow fiber supporting membrane so that pressure on the inside is greater than pressure on the outside (inside pressure)>(outside pressure), and the pressure difference is 1 kPa to 500 kPa, going through a step for contacting a second solution containing the other of the first monomer and the second monomer with the liquid film of the first solution so as to carry out interfacial polymerization, wherein the pressure difference is maintained when carrying out the interfacial polymerization. at least one or more types of a first monomer selected from polyfunctional amines, and at least one or more types of a second monomer selected from the group consisting of polyfunctional acid halides and polyfunctional isocyanates, the membrane area of the hollow fiber bundles is 1 m 2 or more, and the coefficient of variation of average thickness of the separation active layer in the radial direction and lengthwise direction of the hollow fiber bundles, as calculated according to a method as described in more detail herein consisting of measuring the mass of the separation active layer portion in a scanning electron microscope image of a cross-section of the separation active layer in the direction of thickness, is 0% to 60%; the method comprising going through the steps of: forming a liquid film of a first solution containing either the first monomer or the second monomer on the inner surface of a microporous hollow fiber supporting membrane, providing a pressure difference between the inside and outside of the microporous hollow fiber supporting membrane so that pressure on the inside is greater than pressure on the outside (inside pressure)>(outside pressure), and the pressure difference is 1 kPa to 500 kPa, going through a step for contacting a second solution containing the other of the first monomer and the second monomer with the liquid film of the first solution so as to carry out interfacial polymerization, wherein the pressure difference is maintained when carrying out the interfacial polymerization. forming a liquid film of a first solution containing either the first monomer or the second monomer on the inner surface of a microporous hollow fiber supporting membrane, providing a pressure difference between the inside and outside of the microporous hollow fiber supporting membrane so that pressure on the inside is greater than pressure on the outside (inside pressure)>(outside pressure), and the pressure difference is 1 kPa to 500 kPa, going through a step for contacting a second solution containing the other of the first monomer and the second monomer with the liquid film of the first solution so as to carry out interfacial polymerization, wherein the pressure difference is maintained when carrying out the interfacial polymerization.