Peripheral pulmonary arterial aneurysm has rarely been reported, and although most cases show symptoms of hemoptysis. We experienced a case of idiopathic peripheral pulmonary arterial aneurysm in the asymptomatic period noted on medical examination. The patient was a 49-year-old man with coin lesion which was 15 mm in diameter in the right lower lung field on chest X-ray film at the 1st visit. It was strongly stained on enhanced computed tomography in the right S4 and continuity with the pulmonary arteries was suggested. The right middle lobectomy was performed under the diagnosis of peripheral pulmonary arterial aneurysm. Histological examination of the resected specimen revealed that the prominent thinning in the arterial wall, however, there was no evidence of specific chronic inflammation. The postoperative course was good. The patient has been well without new pulmonary arterial aneurysm for 2 years after the surgery.