A hydrophone using a liquid delay line oscillator has been developed. The delay line consists of two pairs of interdigital transducers on the same substrate surface and a diaphragm used as a reflecting plate of the sound beam. The interdigital transducer operates as a sound beam radiator or a detector at a liquid‐solid interface. The oscillation frequency is dependent upon the sound pressure level applied to the diaphragm. Performances of the hydrophone are described, including sensitivity and its frequency dependence.