To reduce a load required for alignment and to suppress flickering of a screen when displaying oxygen saturation.

Positional deviations exist in spectral images PB1-PB3 due to the difference of imaging timing. Contrast conversion processing is applied to the spectral image PB1 to generate a pseudo spectral image PB2' having density histogram similar to the spectral image PB2, and a pseudo spectral image PB3' having density histogram similar to the spectral image PB3. Oxygen saturation is computed using the spectral image PB1 and the pseudo spectral images PB2', PB3' without positional deviations with respect to the spectral image PB1.