This is a case report regarding a 45-year-old woman, who has been undergoing treatment for diabetes mellitus (DM) with chronic thyroiditis (euthyroid state). The patient was admitted to our hospital for the evaluation of a right adrenal tumor (50 x 45 mm) and episodic hypertension. She was diagnosed as having pheochromocytoma based on the increased catecholamine and metabolite concentrations and the result of iodine-131 metaiodobenzyl guanidine ((131)I-MIBG) scintigraphy. Subsequently, the right adrenal tumor was excised. Slowly, progressive type 1 DM (SPIDDM) was confirmed by seropositivity to anti-glutamic acid decarboxylase (1890 U/ml) and the clinical course. After right adrenalectomy, the elevated catecholamine and metabolite concentrations and blood pressure returned to normal, and the dosage of insulin injection was reduced. However, she still needed the insulin injection therapy to control her blood glucose level. This case exhibited an extremely rare combination of pheochromocytoma and SPIDDM with chronic thyroiditis. Although it is common for patients with pheochromocytoma to exhibit glucose intolerance, this case raises the suggestion that measuring the levels of the autoantibody for pancreatic islet cells should be considered if SPIDDM is suspected in a patient with pheochromocytoma.