We report a case of primary central nervous system lymphoma(PCNSL)originating from an optic chiasma, which was difficult to diagnose but was finally diagnosed by biopsy. A 62-year-old immunocompetent man presented with bilateral visual field disturbance, hypopituitarism, and diabetes insipidus;an optic chiasm lesion was detected on MRI. After starting steroid supplementation for adrenal insufficiency, visual field disturbance immediately improved. Since the lesion completely disappeared three months after its onset, it became the follow-up without histological confirmation. Six months after the onset, visual field disturbance progressed, and the lesion recurred. We performed a left optic nerve biopsy to maintain the right visual field, which remained partially. The pathology was PCNSL. We performed postoperative chemoradiotherapy, and the patient showed remission and improvement of the visual field. Isolated PCNSLs arising from optic chiasma are very rare. The diagnosis of optic chiasm lesions is difficult due to their similarity with a variety of inflammatory/autoimmune disease and neoplastic lesions. When a lymphoma is considered to be differentiated, early biopsy should be performed before administering a steroid. The approach and sampling site to prevent the function are also important for biopsy.