A 77-year-old man with a history of surgery for colon cancer in 20XX-3 (year) was referred to our department with suspected primary lung cancer. Computed tomography performed in 20XX-1 (year) revealed the growth of nodules in the right lung. On examination, the left supraclavicular, left axillary, and mediastinal lymph nodes were found to be swollen as well. Based on lymph node biopsy, BRAF mutation-positive lung cancer was diagnosed, and dabrafenib/trametinib were started. Since the right pulmonary nodules continued to grow even after the initial treatment, we biopsied them. Histopathology of the right pulmonary nodule was almost identical to that of the descending colon cancer. Both tumors were KRAS mutation positive. Based on immunohistochemistry (IHC) and genetic testing, the patient was diagnosed with two malignancies: lung metastasis from colon cancer and primary lung cancer. Here, we describe a rare case wherein we were able to differentiate between primary lung cancer and lung metastasis from colon cancer based on genetic testing, IHC, and the therapeutic effects of anticancer drugs.