A 71-year-old male, who had been followed up after being treated with chemo-radiotherapy for non-small cell lung cancer (adenocarcinoma), rapidly developed dyspnea and mild fever. Radiographs showed left pleural effusion and cardiomegaly, and echocardiographic examination revealed echo-free space, suggesting a pericardial effusion. The patient was treated conservatively without any surgical procedures such as pericardiocentesis. Disappearance of the echo-free space was followed by development of pericardial constriction within two months. At post-mortem examination, a direct extension to the pericardium from the primary lesion of the right upper lobe through the mediastinum was observed. The rapid development of pericardial constriction is extremely rare in patients with malignant pericarditis.