We herein report a patient with acute onset type 1 diabetes mellitus that developed three months after delivering a child with depletion of insulin secretion just after the onset. This patient was a 32-year-old woman who recognized excessive thirst, polydipsia, polyuria and digestive symptoms. She visited the emergency room in our hospital, and her laboratory findings showed that her blood glucose level was 664 mg/dL, HbA1c 9.7 %, urine ketone bodies (3+), venous blood gas pH 7.018, fasting-serum C peptide 0.41 ng/mL, and islet cell antibody (ICA) 20 JDF units. We diagnosed her with autoimmune acute-onset type 1 diabetes with diabetic ketoacidosis (DKA). Her HLA-typing was DRB1*04: 05-DQB1*04: 01, which indicates susceptibility to Japanese type 1 diabetes. Her insulin secretion further deteriorated and was almost completely lost just 10 days after the onset. We considered her beta cell destruction to have been accelerated after the onset of acute-onset type 1 diabetes (autoimmune).