A 44-year-old woman was admitted to our hospital with chest pain. Chest roentgenograms and computed tomography (CT) scan revealed an anterior mediastinal tumor and bilateral pleural effusion. However, CT scan 3 days after magnetic resonance imaging (MRI) revealed regression of the tumor. Extended thymo-thymectomy was performed via median sternotomy. The tumor was in the right lobe of the thymus. Pathologically the tumor was diagnosed as a mixed type non-invasive thymoma, but some parts of the tumor cells were necrotic. This is the 10th case of spontaneous regression of thymoma reported in the Japanese literature.