A 50-year-old male was admitted to our hospital with symptoms of fever and gradual development of dyspnea. At that time, he was in a state of acute left heart failure, hypoxic, and with bilateral congestion shadow on chest X ray. Respiratory assistance and infusion of diuretics rapidly improved the left heart failure. Transthoracic echocardiography and cardiac catheterization revealed acute severe aortic regurgitation, but transesophageal echocardiography could not identify the cause of the aortic regurgitation, such as infective endocarditis. We figured that this severe aortic regurgitation was a candidate for valve replacement surgery, so a radical operation for aortic regurgitation was undertaken on the 40th day after the onset of left heart failure in another hospital. During the operation, a localized aortic dissection appeared to be above the left coronary cusp through the right coronary cusp of the aortic valve, so we judged that the cause of aortic regurgitation was this localized aortic dissection. We analyzed the preoperative 3D transesophageal echocardiography findings again, and we verified the presence of a flap lying directly on the right coronary cusp. In addition, we could rebuild the range of localized aortic dissection. Aortic regurgitation due to localized aortic dissection is a relatively rare disorder, and it is often difficult to make a diagnosis of this disorder. We concluded that in cases of acute aortic regurgitation with unidentified cause, suspected on the grounds of the clinical history and transthoracic echocardiography findings, 3D transesophageal echocardiography should be used to make a decision for the purpose of emergency opration.