We report a case of Type-1 diabetes with insufficient serum immunoreactive insulin (IRI) elevation after subcutaneous NPH-insulin injection. Favorable glycemic control was achieved by a continuous subcutaneous insulin infusion (CSII) using regular insulin. A 34-year-old woman with Type-1 diabetes (height 158 cm, weight 43.4 kg) was admitted to our hospital to improve glycemic control. On admission, her glycosylated hemoglobin (HbA(1c)) level was 10.9% and her fasting plasma glucose (FPG) level was 332 mg/dl. After admission, her insulin regimen was altered from two injections a day using premixed insulin to four injections a day using regular insulin before each meal and NPH insulin at bedtime. Although the dosage of NPH insulin at bedtime was increased to 32 U/day, there was no improvement in the FPG level. The peak IRI value after NPH insulin injection was not observed but that after the regular insulin injection was observed. Therefore, her insulin administration regimen was changed to CSII, using regular insulin alone. Her fasting plasma glucose level decreased, glycosylated hemoglobin (HbA(1c)) level improved to 7.0%, her body weight increased to 46.6 kg 4 months after starting CSII.