A pulse signal generator circuit generates first and second pulse signals for electrodes placed on the chest of a subject with phases that differ by 180°. A rectifier circuit receives a potential difference signal, which reflects an impedance between the electrodes that changes according to respiratory motion and in which timing of changes caused by changes in the first pulse signal or the second pulse signal is delayed from the timing of the changes in the first pulse signal and the second pulse signal, and outputs a rectified signal produced by rectifying the potential difference signal. A control signal generator circuit causes the rectifier circuit to invert the potential difference signal, which becomes a negative voltage value from the certain timing, to a positive voltage value from the certain timing. An AD converter circuit outputs a digital value based on the magnitude of the rectified signal.