A sensor includes a substrate, a medium on which a surface acoustic wave propagates, a reference electrode, an antenna, a transducer that includes a first electrode connected to the antenna and a second electrode connected to the reference electrode and arranged opposite to the first electrode, and that has a function of converting a signal received by the antenna into a surface acoustic wave propagating on the medium, and a function of converting a surface acoustic wave propagating on the medium and reaching the transducer into a signal and outputting the signal to the antenna, a reflector that throws back a surface acoustic wave generated by the transducer, propagating on the medium, and reaching the reflector, toward the transducer, and an impedance converter connected to the reference electrode, impedance of which changes in response to a change in circumstances surrounding the sensor.