Acquired hemophilia A (AHA) is a bleeding disorder due to the autoantibody (inhibitor) production targeting blood coagulation factor VIII. It is characterized by a sudden onset, and it often causes extensive and severe bleeding in soft tissue. The incidence of AHA is 1.48 cases per 1 million individuals per year and is common among postpartum women and elderly with underlying diseases. The risk factors include autoimmune diseases, malignancy, and aging. The diagnosis requires exclusion of other diseases with activated partial thromboplastin time (APTT) prolongation and an APTT cross-mixing test during early differential diagnosis. The treatment of AHA is immunosuppressive therapy to reduce the inhibitors. In case of bleeding that requires hemostasis, hemostatic therapy with bypass agents should be administered. The first-line immunosuppressive therapy is prednisolone (1 mg/kg/day) alone or in combination with cyclophosphamide (1-2 mg/kg/day). Recently, the effect of a rituximab-based-regimen has also been utilized.