Cerebrospinal fluid (CSF) overdrainage after lumboperitoneal (LP) shunt placement for the patients with idiopathic normal pressure hydrocephalus (iNPH) is mainly caused by insufficient management of pressure settings of the shunt valve and/or siphon effect of shunt systems induced by the patient's postural changes. We here report a unique case of intracranial hypotension (IH) due to CSF leakage after LP shunt placement in which another mechanism leads to the CSF leakage. A 67-year-old man suffered from persistent headache worsening with postural change 2 months after LP shunt reconstruction for iNPH. Brain computed tomography scan showed bilateral chronic subdural hematomas (CSDH). Lumbar images including shuntography and magnetic resonance imaging showed the tip of the lumbar catheter was spontaneously pulled out close to the dura mater with expansion of the epidural space due to CSF leakage from a shunt side hole of the lumbar catheter to the epidural space. Shunt removal and subsequent irrigation of CSDH improved his headache. CSF leakage in our case differs from those in previous reports, because early and enormous CSF leakage into the epidural space can be explained only by a different mechanism through a side hole just located in the epidural space in our case. We must pay attention to the possibility of this rare cause of IH due to CSF leakage in patients suffering from postural headache after LP shunt placement.