We report a case of diabetic ketoacidosis (DKA) and severe hypertriglyceridemia who developed cardiac arrest due to hypophosphatemia. He was diagnosed with diabetes and hyperlipidemia, indicating metabolic syndrome. Hypophosphatemia was caused by large insulin doses received while treating DKA, which were required because of insulin resistance owing to hypertriglyceridemia. Metabolic syndrome may have accelerated serum phosphate depletion. We suggest frequent monitoring of serum phosphate and phosphate replacement for patients with DKA and severe hypertriglyceridemia. Although such a critical condition has not been reported, it may occur during treatment of patients with poorly controlled type 2 diabetes with DKA.