The patient was a 72-year-old female. She had been taking rivaroxaban for chronic atrial fibrillation; however, she stopped taking it due to anemia and was hospitalized urgently. A contrast-enhanced computed tomography(CT)scan showed a 30 mm mass in the ascending colon, and a colonoscopy revealed ascending colon cancer(cT3, cN0, cM0, cStage Ⅱa). The tumor was hemorrhagic and was thought to have caused the anemia. On day 6 of hospitalization, another contrast- enhanced CT scan showed a poorly contrast-enhanced area in the left atrium, and transesophageal echocardiography revealed 2 left atrial thrombi(27 mm and 17 mm). Since early induction of anticoagulation therapy was considered, an emergency open right colectomy was performed to remove the cause of the bleeding. Intravenous heparin therapy was started the day after surgery and was switched to oral apixaban therapy on the fourth postoperative day. The postoperative course was good, and she was discharged home on the 17th postoperative day. This patient had conflicting clinical problems simultaneously; however, immediate decision-making and initiation of treatment were effective.