Objective: To report on septic shock in a patient with an abscess inside cystic adenomyosis after responsive treatment of pelvic inflammatory disease. Case report: A 51-year-old woman with known cystic adenomyosis was admitted to the hospital due to fever and low abdominal pain. In suspicion of pelvic inflammatory disease, intravenous antibiotics were administered. As her symptoms and sings of inflammation improved, she was discharged from the hospital after five days. However, after completing 14 days of oral antibiotics, her symptoms aggravated with decreased blood pressure. Magnetic resonance imaging indicated abscess formation inside cystic adenomyosis, and total hysterectomy was performed. Postoperatively, she fell into shock condition. Intravenous antibiotics were administered for 11 days and she was discharged from the hospital with no adverse event. Conclusion: Physicians should be aware of the possibility of septic shock caused by abscess formation inside known cystic adenomyosis.