Thymic carcinomas is rare and highly aggressive carcinoma. Most patients with them are diagnosed as being at surgically unresectable stages due to it. There are several reports which showed the effect of chemotherapy, however, it is controversial. Recently, immune checkpoint inhibitors have changed conventional chemotherapy due to their effect against various types of cancers. We administered nivolumab, anti-Programmed Cell Death (PD)-1 antibody, to four patients with unresectable thymic carcinomas who had previously undergone conventional chemotherapy. A histopathology on tumors from these patients revealed the presence of squamous cell carcinoma and PD-L1 high expression. After treatment with nivolumab, it seemed to be beneficial to all patients; The best clinical responses of 3 patients were partial response and that of the other one was stable disease. None of them experienced severe immune-related adverse events. Our results suggest the potential benefits of using these inhibitors to treat thymic carcinomas in real world clinical setting as is the cases in recent clinical trials for the evaluation of immune checkpoint inhibitors for the treatment of thymic carcinoma.