An 85-year-old man presented with right eye ptosis and visual abnormalities. Due to a lack of abnormal findings on plain magnetic resonance imaging (MRI) and laboratory examination, prednisolone therapy was initiated, and ocular myasthenia gravis and ocular symptoms subjectively improved. However, anorexia and conscious disorder appeared during the same period, and he was hospitalized for scrutiny. After admission, left eye adduction disorder and bilateral abduction nerve paralysis were also observed. Enhanced MRI revealed cranial nerve and leptomeninx enhancement. Subsequently, the patient developed seizures and died on the 10 day of hospitalization without being diagnosed. An autopsy revealed infiltration of B-cell lymphoma cells into the subarachnoid space, particularly in the parietal lobe. Similar infiltration was observed in the cerebellum, brainstem, spinal cord, and bilateral oculomotor nerve. Primary leptomeningeal lymphoma is a rare form of primary central nervous system lymphoma without simultaneous parenchymal brain lesions. Clinicians should be aware of this form of lymphoma and carefully monitor its possible occurrence, even when patients are already being treated for other neurological diseases.