A 69-year-old man with type 2 diabetes mellitus was admitted to our hospital because of appetite loss, nausea and vomiting. Gastroscopy revealed gastric cancer. Levels of plasma cortisol were decreased. Neither adrenocorticotropic hormone (ACTH) nor cortisol levels were adequately increased in response to a mixed intravenous administration of corticotropin-releasing hormone, growth hormone-releasing hormone, thyrotropin-releasing hormone and lutenizing hormone-releasing hormone, although other pituitary hormones were increased adequately. He was diagnosed as having isolated ACTH deficiency (IAD). Anti-pituitary antibody and anti-parietal cell antibody were positive. At least in part, these antibodies may play pathogenic roles of development of IAD and gastric cancer.