A sound wave sensor, capable of reliably detecting presence or absence of a banknote very easily with use of sound waves, is provided. The sound wave sensor includes a sounding body 2 that outputs a sound wave signal of a predetermined frequency, an echo body 3 opposed to the sounding body 2 in a direction in which the sounding body 2 outputs the sound wave signal, the echo body 3 being provided at a position where the sound wave signal resonates at the predetermined frequency; a slit 5 through which a sheet-like banknote 4 passes, the slit 5 being provided between the sounding body 2 and the echo body 3; and a resonance determination unit 6 that determines a resonant state of the frequency output from the sounding body 2. The resonance determination unit 6 determines a resonant state by detecting variation in power consumption of the sounding body 2.