The incidence, morbidity, and mortality of colorectal cancer are increasing, largely owing to an increasingly aging population. Additionally, along with the increasing age of cancer patients, the number of patients with various comorbidities such as membranous nephropathy is also rising, and problems associated with the administration of chemotherapy to elderly patients with these conditions are becoming more common. Herein, we describe a case involving an 80-year-old woman who presented with general malaise, edematous limbs, and pleural effusion. An abdominal CT revealed multiple, relatively large, metastatic lesions in a wide area of the liver and left pleural effusion, and she was accordingly diagnosed with membranous nephropathy secondary to ascending colon cancer and multiple liver metastases. Despite her advanced age and the presence of membranous nephropathy, her general condition was favorable and chemotherapy was hence administered. Taking the toxicity profiles and the patient's preference into consideration, S-1 and oxaliplatin (SOX) therapy was selected, which showed a good tolerability. An abdominal CT after 8 cycles of SOX therapy revealed a marked reduction in the metastatic lesions in the liver and a decrease in the left pleural effusion, and the levels of tumor markers also decreased (partial response). At the latest follow-up, after the completion of 16 cycles, the condition of the patient remained stable, without any apparent signs of progressive disease. Based on this case, we conclude that, even for elderly patients with systemic complications or comorbid diseases, standard treatments should be considered after their general conditions, and therapeutic regimens have been sufficiently examined.