An 18-year-old male with huge anterior mediastinum mass was scheduled for thoracotomic incisional biopsy under general anesthesia after failed fluoroscopy-guided percutaneous needle biopsy. Under propofol and succinylcholine anesthesia, intubation was successfully achieved using a Univent tube. However, when we changed the patient's position from supine to right lateral decubitus, oxygen saturation declined. He was then positioned supine, but hypoxemia did not improve. Because the tumor expanded toward the left thoracic field, we considered that the left lateral decubitus position might help relieve the mass effect on the main bronchus. His position was changed accordingly and soon after, hypoxemia improved and surgery was undertaken under cardiopulmonary bypass (CPB). The biopsy was successfully performed under CPB without complication.