A 26-year old Japanese woman experienced new aortic valve regurgitation associated with a preceding high fever of unknown cause. During the fever episode, although bacteremia or fungemia was not evident despite frequent blood cultures, intravenous panipenem/betamipron (PAPM/BP) gradually resulted in decline of the fever. Echocardiography and operative procedures revealed a quadricuspid aortic valve (QAV), which was composed of two equal larger cusps and two unequal smaller cusps (type f). A smaller accessory cusp was damaged but showed no active vegetation. A Medtronic Freestyle bioprosthesis was implanted using a subcoronary technique. Although the risk of endocarditis for this rare valve abnormality is not well documented, the present case may support the conventional assumption that patients with unequal small cusps are prone to endocarditis.