A 70-year-old woman had consulted a doctor at a former clinic because of bloody stool and colonoscopy revealed a type 2 tumor of the rectum. She was referred to our hospital for further examinations and treatment. Preoperative blood examination showed an elevated HbA1c level of 10.2%. Abdominal CT showed a 25mm tumor in the left adrenal gland. The patient was diagnosed with adrenal Cushing's syndrome based on low ACTH levels, disappearance of circadian variation in blood cortisol levels, lack of inhibition by dexamethasone loading, and high urinary cortisol levels. Laparoscopic adrenalectomy for left adrenal tumor and low anterior resection for rectal cancer were performed. The pathological findings were rectal cancer, pap, pT1b(SM), pN0, cM0, fStageⅠof rectal cancer, and adrenal cortical adenoma. The postoperative course was uneventful with steroid replacement therapy. The ileal stoma was closed 4 months after surgery. Surgery in hyperadrenalism requires perioperative steroid replacement therapy because of the risk of postoperative acute adrenal failure. In addition, when diabetes is poorly controlled, we should be careful about risk of leakage and susceptibility to infection.