A 60-year-old man with neovascular glaucoma due to diabetic retinopathy received an intravitreal injection of 1.25 mg bevacizumab (IVB) followed by extensive panretinal photocoagulation in the right eye. The anterior segment neovascularization regressed within 10 days after IVB. One and a half months later, the patient underwent gastrectomy for stage IIIb gastric cancer. Two months later, he was started on S-1 orally (100 mg/day for 48, 26, and 32 consecutive days in the first, second, and third treatment cycle, respectively). The interval between the first and second treatment cycle was 20 days and between the second and third cycle it was 24 days. The patient developed anemia and diarrhea. At the end of the second S-1 cycle, cystoid macular edema developed in the right eye, although diabetic retinopathy and neovascular glaucoma were stable. Macular edema persisted for 5 months despite another IVB, and disappeared 3 months after termination of S-1 therapy. The time course of the magnitude of macular edema correlated well with the severity of anemia. The macular edema was possibly associated with anemia, which is a major side effect of S-1. Further studies are warranted to investigate the relationship between anemia and macular edema in patients with diabetic retinopathy.