A P4 segment aneurysm of the posterior cerebral artery has rarely been described. A case of ruptured P4 segment aneurysm, which re-ruptured after clipping procedure for unruptured internal carotid artery aneurysm, was reported. A 57-old-man had sudden onset of severe headache and vomiting and was transferred to our hospital. CT scan on admission showed diffuse subarachnoid hemorrhage dominantly extending to the tentorial surface and the occipital interhemispheric tissue. Four-vessel angiography demonstrated a right internal carotid-posterior communicating artery junction aneurysm, and its neck clipping was performed on day 5. Intraoperative inspection of the whole appearance of the aneurysm was difficult because of the aneurysm existing on the ventral portion of the internal carotid artery and definite diagnosis of the bleeding source was not obtained. On day 23, he complained of severe headache and restricted vision and CT scan showed intracerebral hematoma in the left occipital lobe with intraventricular hemorrhage. The angiograms and CT scan on admission were reexamined, and another aneurysm on the left parieto-occipital artery (P4 segment) was retrospectively identified. The ruptured P4 segment aneurysm was obliterated via the interhemispheric approach and the patient enjoyed an uneventful postoperative course. When a thick subarachnoid hemorrhage distributed in the occipital interhemispheric fissure, quadrigeminal cistern, and ambient cistern is encountered, the existence of a possible P4 segment aneurysm should be suspected. Correct initial diagnosis and definite treatment of the ruptured lesion in the acute stage is essential in dealing with SAH-patient with multiple aneurysms. When they are unruptured lesions at a common aneurysm site, the existence of an unusually located aneurysm should not be overlooked as the possible source responsible for symptoms.