Bacterial aneurysm of the common carotid artery is rare. Septicemia, dental infection, postoperative infection, and endocarditis are thought cause this disease. In this paper, we describe a case of a common carotid artery bacterial aneurysm caused by neck lymphadenitis.A 49-year-old man was referred to us for treatment of pain in the third molar region of the left side of the mandible and swelling and pain in the left side of the neck. After the symptoms resolved in response to antibiotic chemotherapy, we extracted the left lower second molar and impacted third molar. Twelve days after, he visited our hospital again because of swelling in the left side of the neck. Pulsation could be noted on the swelling. The white blood cell count was 14880/ml, and the C-reactive protein level was 10.77mg/dl. A CT scan revealed a pseudoaneurysm of the left common carotid artery. He was referred to neurosurgeons and received a diagnosis of aneurysm of the common carotid artery caused by bacterial infection, judging from the clinical course. Antibiotics were administered for 6 weeks. After the inflammation resolved, he underwent common carotid-internal carotid artery bypass. Histopathologically, the diagnosis was bacterial aneurysm caused by neck lymphadenitis.