A report is presented on two cases of chronic subdural hematoma which occurred after craniotomy, removal of the hematoma and external decompression for traumatic intracranial hematoma. As for the pathogenesis of chronic subdural hematoma of these two cases, these chronic subdural hematomas were considered to have originated from the subdural collection of mixed fluid of cerebrospinal fluid and blood caused by tearing of the arachnoid on head injury or craniotomy in the space between the brain and dural plasty. Opening and irrigating the hematoma cavity proved adequate as treatment at cranioplasty.