We describe a patient with isolated inferior rectus palsy due to midbrain infarction. A 68-year-old-man developed vertical diplopia of sudden onset which was maximal with a right lower side gaze. Neurological examination revealed weakness of the right inferior rectus muscle. Magnetic resonance imaging of the brain showed a focal infarction in the dorsomedial portion of the right midbrain tegmentum. The sole involvement of fascicular fibers of the inferior rectus muscle in the midbrain was accountable as the cause of this isolated inferior rectus palsy. We think that this is a case supporting the three-dimensional model of somatotopic fascicular arrangement of oculomotor nerve in the midbrain.