A 62-year-old woman was referred to our hospital because an abnormal shadow was found in the left upper lobe by computed tomography of the chest. Contrast-enhanced CT showed that the superior pulmonary vein did not drain into the left atrium, but rather into the left branchiocephalic vein. The patient had a pulmonary-to-systemic flow ratio (Qp/Qs) of 1.2, and there was no reason to repair the partial anomalous venous connection. Primary lung cancer was suspected, and surgery was performed. Intraoperatively, the anomalous pulmonary vein was seen to drain into the left brachiocephalic vein. The lesion was resected and diagnosed as primary adenocarcinoma, and left upper segmentectomy and mediastinal node resection were performed. Partial anomalous venous connection with lung cancer is rare, and most cases are discovered intraoperatively. We discovered the anomalous venous connection preoperatively and were able to perform the operation safely.