A case was 73-year-old man, who had history of laparoscopic high anterior resection surgery for rectal cancer, followed by adjuvant chemotherapy 2 years ago. Preoperative diagnosis was anterior mediastinal tumor, with multiple intrapulmonary nodules noted, though no increasing tendency. During adjuvant chemotherapy for colorectal cancer, the anterior mediastinal tumor showed some shrinkage, while that and 3 intrapulmonary nodules slowly increased in size after completion, thus rectal cancer pulmonary and mediastinal metastasis were suspected. Complete resection of the intrapulmonary nodules and anterior mediastinal tumor was considered feasible. Thoracoscopic observation revealed multiple small pleural seeding lesions and all speculated to be intrapulmonary metastases before surgery were also pleural lesions. Intraoperative rapid diagnostic findings of a biopsy section revealed possible colorectal cancer metastasis, though histological type was not revealed. Final histopathological diagnosis was pleural dissemination of thymic carcinoma. Lenvatinib was introduced 2 months later for thymic carcinoma with pleural dissemination. Two years after surgery, the anterior mediastinum primary tumor had slightly decreased and the pleural nodules also showed a shrinking tendency. In such cases of small tumor with increasing tendency and irregular margins, thymic carcinoma should be considered when planning treatment.