A Fourier transform infrared spectrophotometer, comprising: a) a light source adapted to generate infrared light having a wavelength width including an absorption wavelength of a compound to be analyzed; b) an interferometer including a fixed mirror and a movable mirror, adapted to generate interfering light from the infrared light; c) a detector adapted to generate a voltage with a magnitude corresponding to an intensity of the interfering light, and to output a voltage obtained by subtracting, from the aforementioned voltage, a voltage with a predetermined magnitude; d) a polarity inverter adapted to output a voltage obtained by inverting a polarity of an input voltage; e) a bias voltage adder adapted to add a bias voltage with a predetermined magnitude to an input voltage; f) an amplifier adapted to amplify an input voltage by a predetermined multiplying factor; g) an analogue-to-digital converter adapted to convert an input signal into a digital signal; h) a first signal processor adapted to perform a first signal-processing operation in which an output voltage from the detector is successively processed by the bias voltage adder and the amplifier; i) a second signal processor adapted to perform a second signal-processing operation in which an output voltage from the detector is successively processed by the polarity inverter, the bias voltage adder, and the amplifier; j) a judging section adapted to input, into the analogue-to-digital converter, each of the voltages obtained through the first signal-processing operation and the second signal-processing operation, and to make a judgment on whether or not each of the voltage falls within an input range of the analogue-to-digital converter; and k) a signal-processing determining section adapted to determine which of the first and second signal-processing operations should be used to process an output voltage from the detector in an actual measurement, based on a result of the judgment by the judging section. a) a light source adapted to generate infrared light having a wavelength width including an absorption wavelength of a compound to be analyzed; b) an interferometer including a fixed mirror and a movable mirror, adapted to generate interfering light from the infrared light; c) a detector adapted to generate a voltage with a magnitude corresponding to an intensity of the interfering light, and to output a voltage obtained by subtracting, from the aforementioned voltage, a voltage with a predetermined magnitude; d) a polarity inverter adapted to output a voltage obtained by inverting a polarity of an input voltage; e) a bias voltage adder adapted to add a bias voltage with a predetermined magnitude to an input voltage; f) an amplifier adapted to amplify an input voltage by a predetermined multiplying factor; g) an analogue-to-digital converter adapted to convert an input signal into a digital signal; h) a first signal processor adapted to perform a first signal-processing operation in which an output voltage from the detector is successively processed by the bias voltage adder and the amplifier; i) a second signal processor adapted to perform a second signal-processing operation in which an output voltage from the detector is successively processed by the polarity inverter, the bias voltage adder, and the amplifier; j) a judging section adapted to input, into the analogue-to-digital converter, each of the voltages obtained through the first signal-processing operation and the second signal-processing operation, and to make a judgment on whether or not each of the voltage falls within an input range of the analogue-to-digital converter; and k) a signal-processing determining section adapted to determine which of the first and second signal-processing operations should be used to process an output voltage from the detector in an actual measurement, based on a result of the judgment by the judging section.