A 78-year-old man presented to our hospital with lung abnormality on his chest radiograph. Computed tomography (CT) showed a mass and obstructive pneumonia in the right upper lobe of the lung. The mass was diagnosed as a pulmonary adenocarcinoma with a bronchoscopy (cT4N2M0, Stage IIIB). CT also revealed multiple hepatic tumors, which were diagnosed as hepatocellular carcinoma (HCC) by dynamic CT and gadolinium ethoxybenzyl diethylenetriamine pentaacetic acid enhanced magnetic resonance imaging(EOB-MRI). First, we treated the lung cancer with a combination of cisplatin and pemetrexed (PEM), but it caused renal dysfunction. Carboplatin (CBDCA) and PEM combination chemotherapy was administered, and not only the lung cancer but also the HCCs decreased in size. There are few reports of synchronous double cancers of HCC and primary lung cancer, and the treatment is not established. We report that platinum-containing anticancer drugs such as CBDCA may be effective against synchronous double cancers of HCC and lung cancer.