Pure motor monoparesis (PMM) is a rare condition characterized by weakness limited to one limb without sensory disturbance. We report a 42-year-old woman with PMM of the right lower limb caused by a small infarction in the contralateral motor cortex that could be detected by the magnetic resonance imaging of the brain. This case suggests that small lesions, missed by carelessly performed scans, could be a potential cause of PMM. This is especially true in the case of lower limb PMM, because the lesion may be located in the top of the frontal lobe cortex, an area that can be easily missed by routine scans. Therefore, we should pay careful attention to the opposite side of the motor cortex in examining neuroimages of PMM cases.