A surface-acoustic-wave-sensor-included oscillator circuit does not cause separation of an electrode film due to application of a bias voltage and can reliably accurate operate even if liquid is adhered thereto. The surface-acoustic-wave-sensor-included oscillator circuit includes interdigital electrode disposed on a piezoelectric substrate and a reaction film that is arranged so as to cover the interdigital electrodes and bound to a target substance or a binding material to be bound to the target substance. A surface acoustic wave sensor that is capable of detecting a bit of mass loading on the basis of a variation in frequency is connected as a resonator in the surface-acoustic-wave-sensor-included oscillator circuit. Direct-current cutting capacitors are connected in series to the surface acoustic wave sensor, and the direct-current cutting capacitors respectively define impedance matching circuits in the surface-acoustic-wave-sensor-included oscillator circuit.