We report herein on a case of interdigitating dendritic cell sarcoma (IDCS) of a neck lymph node. A 43-year-old man was seen for left neck swelling. The left neck was found to have a smooth and soft tumor. We found atypical cells with fine needle aspiration cytology, and performed a cervical lymph excisional biopsy. Diagnosis was confirmed by histology and immunostaining. After the lymph node biopsy, additional radiation therapy was performed. Generally, diagnosis of IDCS is difficult. In this case, the tumor lesion was limited to a cervical lymph node and fine needle aspiration cytology suggested the possibility of malignancy. Treatment resulted the patient’s being desease-free at 18 months after radiation therapy.