To obtain an oxygen saturation image having a resolution as high as possible while ensuring a sensitivity required to capture changes in degree of oxygen saturation of hemoglobin in blood.

Illumination light for oxygen saturation of 473±10 nm having a different absorption coefficient for oxyhemoglobin and deoxyhemoglobin is used to generate an oxygen saturation image. When the amount of the illumination light for oxygen saturation is above the predetermined threshold value, the illumination light for oxygen saturation is detected by a monochromatic ordinary sensor 53. When the amount of the illumination light for oxygen saturation is below the predetermined threshold value, the illumination light for oxygen saturation is detected by a monochromatic high-sensitivity sensor 60.