A 73-year-old male was referred to our hospital for evaluation of a lung tumor that had been detected by chest computed tomography (CT) during a follow-up examination for colon cancer. CT showed reticular shadows in both peripheral lung fields. In addition, pure solid nodular shadowing and ground glass opacity (GGO) were seen in the left upper lobe. Partial resection of the tumor in the left upper lobe was performed on the suspicion of metastatic lung cancer. Histopathological examinations revealed the nodular lesion to be a primary lung squamous cell carcinoma and the GGO to be a primary lung adenocarcinoma. He was diagnosed with primary pulmonary collision cancer comprising squamous cell carcinoma and adenocarcinoma. It is difficult to diagnose a collision cancer of the lung before surgery, especially in patients with interstitial lung disease. Detailed histopathological retrieval of the resected specimen is important to obtain an accurate diagnosis of collision cancer.