A 78-year-old man presented to a local doctor with dyspnea. A chest radiogram revealed a tumor mass 8 cm in longest diameter in the left-middle and lower-lung field. The patient was referred to our hospital. Chest CT showed a convex-shaped tumor about 10 cm in diameter in the region extending from the ventral pleural surface of the left-upper lobe toward the central part, making a differential diagnosis from pleural tumor difficult. A CT performed in the prone position subsequently showed pleural effusion in the space between the lesion and the chest wall, though this fluid collection had not been observed on the previous CT in the supine position. Based on the CT findings, we considered this lesion to be a lung tumor. As part of the preoperative evaluation, the patient underwent thoracoscopy under local anesthesia, which showed a tumorous lesion extending from the ventral left-upper lobe to the pleural cavity. He underwent surgery for differential diagnosis and treatment. The patient was diagnosed with the blastomatoid variant of pulmonary carcinosarcoma consisting of high-grade adenocarcinoma of fetal lung type (H-FLAC) and rhabdomyosarcoma.