Spectral apparatus includes sensors that output signals indicating the respective energy amounts of a plurality of wavelength components. For each of the sensors, its reference spectral sensitivity is acquired, and an indicator indicating the reference spectral sensitivity of the sensor, is acquired. The deviation of an indicator indicating the spectral sensitivity of the sensor from the indicator indicating the reference spectral sensitivity of the sensor, is expressed by a linear function of an indicator indicating a mechanical error in the spectral apparatus. The indicator indicating the mechanical error in the spectral apparatus, is acquired to adapt a spectral sensitivity set of sensors to a signal set output by the sensors. For each of the sensors, the deviation of the indicator indicating the spectral sensitivity of the sensor from the indicator indicating the reference spectral sensitivity of the sensor, is acquired, so that the spectral sensitivity of the sensor is acquired.