A male patient in his 70s was referred to our hospital with the chief complaint of anorexia. Abdominal computed tomography (CT) showed a 20 cm large homogeneous mass in the retroperitoneum, and contrast-enhanced CT revealed uniform staining throughout the inside of the mass. Soluble interleukin-2 receptor and lactate dehydrogenase tumor markers were elevated. Hence, malignant lymphoma was suspected, and ultrasonography-guided biopsy was performed. Histopathological findings showed large lymphocytes with poorly differentiated cytoplasmic nucleoli and positivity for CD20 and CD79a via immunohistochemical analysis, which was consistent with diffuse large B-cell lymphoma. The patient received R-THP-COP therapy which consisted of rituximab, pirarubicin, cyclophosphamide, vincristine and prednisolone. After four chemotherapy courses, a partial response according to the the response evaluation criteria in solid tumors was obtained. The patient was discharged with no signs of recurrence.