Abstract: A method of producing a sulfonated polyarylether block copolymer is provided. The method includes producing a sulfonated polyarylether block copolymer containing a hydrophobic segment having a structural unit represented by formula (5) and a hydrophilic segment having a structural unit having a sulfonic acid groups or derivative thereof incorporated into a structure represented by formula (6). A hydrophilic segment prepolymer having a sulfonic acid group in a potassium salt form and a hydrophobic segment prepolymer are block copolymerized. A proton conductor that includes the sulfonated polyarylether block copolymer is also provided.