We report the case of a 51-year-old woman whose body weight was increased after bariatric surgery. She has been obese from a young age. When she was 30 years old, she developed type 2 diabetes mellitus. She was hospitalized for diabetic ketoacidosis at 45 years of age. She was diagnosed bulimic at 48 years of age, and body weight reduction with diet and exercise therapy became difficult because she was bulimic. Then, she underwent bariatric surgery and her body weight had reduced by 11 kg, 6 months after the surgery. However, her body weight began to increase again 7 months post surgery. At this time, her mental status had become unstable and she ate constantly in the afternoons. We administered yokukansan 5 g/day to treat her unstable mental status. Her body weight then decreased in accord with decreasing energy intake after the administration of yokukansan. She also became aware that her mental status was improving. Furthermore, her HbA1c (JDS) decreased from 8.7% to 7.1% after yokukansan treatment. In this case, it is possible that yokukansan suppressed the overeating by stabilizing her mental status, with the parallel decreases in body weight and HbA1c.