A 78-year-old female fell and hit the back of her head on the floor. Head computed tomography (CT) showed right acute interhemispheric subdural hematoma (ISDH). Her left hemiparesis worsened, so partial removal of ISDH was performed. The hemiparesis was improved, but leg monoparesis persisted. Lumbar magnetic resonance imaging showed spinal subdural hematoma (SSDH) at the S1-2 level. Nerve conduction velocity measurements at the knee joint to lower limb showed disappearance of the left peroneal nerve conduction wave, indicating that one of the causes of drop foot was common peroneal nerve palsy. With conservative therapy, her drop foot was gradually improved, then she recovered to walk with a stick and moved to a rehabilitation hospital. Lumbar MR imaging should be performed to rule out SSDH in a patient with posterior fossa subdural hematoma on initial head CT who develops leg palsy.