A 60-year-old man with unresectable colon cancer required treatment for hydronephrosis, acute cholecystitis, and obstructive jaundice before chemotherapy. His performance status (PS) gradually deteriorated to PS 4. Cetuximab monotherapy was initiated instead of intensive chemotherapy. His general condition improved and mFOLFOX7 therapy was then continued in addition to cetuximab. A computed tomography (CT) scan after 6 months of chemotherapy revealed a partial response (PR). Cetuximab monotherapy may contribute to the treatment of patients with a poor PS.