Leptomeningeal metastasis is a devastating complication of advanced stage cancer. It is frequently accompanied by hydrocephalus and intracranial hypertension that must be treated by ventriculoperitoneal shunts. However, there are actual risks of peritoneal seeding or accumulation of malignant ascites after the cerebrospinal fluid diversion procedure, though it has not been reported. Here, we present the case of a patient with non-small cell lung cancer with leptomeningeal metastasis in whom malignant ascites developed after a subduroperitoneal shunt.