A 72-year-old female was admitted with complaints of cough and sputum. The chest X-ray film revealed a solitary round mass and pleural effusion in the left lower lung field. Laboratory tests demonstrated elevated levels of serum HCG and beta-HCG. Left lower lobectomy with parietal pleurectomy was performed under the diagnosis of primary lung cancer with malignant effusion. The serum HCG level decreased after the operation. Histologically, the tumor was diagnosed as moderately differentiated papillo-tubular adenocarcinoma of the lung. In the HCG staining using an immunohistochemical method, the tumor cells showed a positive reaction. Thus, this tumor was definitively diagnosed to be HCG-producing adenocarcinoma of the lung.