Actinomycosis is a rare disease caused by the Gram-positive anaerobic filamentous bacterium, Actinomyces israelii. Due to its unusual clinical presentation, it is mainly diagnosed postoperatively. The risk of the disease increases with the use of intrauterine contraception device (IUD). A 46-year-old female was admitted to the hospital due to diffuse lower abdominal pain and had used an IUD until two weeks ago. Findings on abdominal computed tomography showed a large mass measuring approximately 9 cm, which was suspected of urachal cancer or omental sarcoma. After wide surgical resection, pathologic finding showed severe inflammation with sulfur granule, confirming abdominal actinomycosis.