<p>Provided is a sensor for detecting a substance in a liquid, by which the substance to be detected in the liquid can be accurately measured at a higher sensitivity. A sensor (1) for detecting a substance in a liquid is provided with a sensing circuit (2) which includes a sensing SAW element (4) wherein a reactive film which reacts to the substance in the liquid is arranged to cover an IDT; a reference circuit (3) which includes a reference SAW element (6) having the IDT but not a reactive film; a first signal source (8) for driving the sensing circuit (2); a second signal source (9) which drives the reference circuit (3) and is separately configured from the first signal source; and a differential circuit (10) for outputting differential output of the sensing circuit (2) and the reference circuit (3). The frequency of the first frequency signal outputted from the first signal source (8) is different from that of the second frequency signal outputted from the second signal source (9). Thus, the driving frequency of the sensing SAW element (4) and that of the reference SAW element (6) are permitted to be same or different.</p>
