Method for producing cation exchange resin and method for refining organic acid solution

A method for purifying an organic acid solution, the method comprising:

• bringing a mineral acid solution having a metal impurity content of 1 mg/L or less and a concentration of 5 mass % or more into contact with a cation exchange resin (A) to obtain a cation exchange resin (B);
• and
• bringing an organic acid solution having a metal impurity content of 2 mg/L or less and a concentration of 3 mass % or more into contact with the cation exchange resin (B) to purify the organic acid solution,
• wherein the cation exchange resin (B) after use of the cation exchange resin (B) for purifying the organic acid solution is reused as the cation exchange resin (A), and
• wherein bringing the mineral acid solution into contact with the cation exchange resin (A) and bringing the organic acid solution into contact with the cation exchange resin (B) are further repeated one or more times.