An exhaust system that processes exhaust generated by an engine is provided. A fundamental wave BPF unit and a fundamental wave detecting unit extract a fundamental wave component from the echo signal, and calculate its signal power. A similarity value calculation unit compares the fingerprint image newly acquired from the user and the fingerprint image held in the data holding unit, and indicates a degree of similarity therebetween as a similarity value by using a similarity evaluation function for indicating a degree of similarity between a plurality of types of fingerprint images. A flexible central support depends laterally inwardly from the outer rim and includes opposed upper and lower surfaces.