A 65-year-old man experiencing chest oppression was admitted to our hospital. Contrast-enhanced chest computed tomography (CT) showed a 71×52-mm cystic mass with an enhanced wall in the thymus. The CT-guided needle biopsy of the tumor only revealed the presence of necrotic tissue. However, the tumor spontaneously decreased in size significantly to 33×21 mm in 2 months. The patient underwent right hemi-lobectomy of the thymus, which showed necrotic tissue in the center of the mass;the fibrotic cystic wall included several masses of type B3 thymoma cells( so called" well-differentiated thymic carcinoma", Masaoka stage II). Because of avoiding local recurrence, extended thymectomy including lymph node resection was additionally performed. The patient received radiation therapy postoperatively and has shown no sign of recurrence in 3 years of follow-up care.