The subject was a 28-year-old woman with fulminant type 1 diabetes mellitus. Her HbA₁c level was maintained at 5 to 6% by intensive insulin therapy with NPH insulin and insulin lispro. The NPH insulin was changed to insulin glargine when the patient was 30. Two years after the onset of her fulminant type 1 diabetes mellitus, the patient became pregnant. Even with HbA₁c level of 5.6%, her plasma glucose was brittle. In the sixth week of pregnancy, continuous subcutaneous insulin infusion (CSII) of insulin lispro was commenced. The CSII successfully stabilized her plasma glucose. The insulin dose was increased later in the course of the pregnancy. In the 37th week of pregnancy, the unborn fetus developed complications and was delivered by a Caesarean section. The newborn was a healthy boy with a birth weight of 3,014 g. Both the mother and son have been doing well since the procedure. CSII may help the prognosis of fetuses in pregnant mothers with fulminant type 1 diabetes pregnancy with brittle control of the plasma glucose.