PROBLEM TO BE SOLVED: To provide a method for producing low-salt soy sauce in which an ion exchange membrane exhibits only a small lowering of performance when run for a long period of time, because a membrane with excellent stain resistance is used.SOLUTION: A method for producing low-salt soy sauce by lowering the salt content in a soy sauce by means of electrodialysis, is characterized in that a cation-exchange membrane that contains a vinyl alcohol copolymer having an anionic polymer segment with anionic group and a vinyl alcohol polymer segment, and that has a micro phase separation structure whose domain size (X) is within a range of 0nm
