A 62-year-old man visited our outpatient department due to a 2.0-cm pulmonary nodule with irregular margins in the left perihilar region that was found on chest x-ray at another institution; however, the patient had experienced no discomfort. This case was a special histological type of synchronous primary lung cancer with adenosquamous carcinoma at the left lower lobe and mixed small cell carcinoma and squamous cell carcinoma at the right lower lobe. Our experience with this case highlights the importance of distinguishing between multiple primary lung cancer and distant lung metastasis.