A liquid sensor has comb-shaped electrodes, applied to a piezoelectric substrate, as interdigital transducers for generating an electroacoustic wave from a transmitting transducer to a receiving transducer. The number of finger pairs of the interdigital transducers and the material out of which the electrodes are made are so chosen that a surface shear wave generated by the transmitting transducer and a bulk shear wave generated by the transmitting transducer have different frequencies.