In imaging an oxygen saturation level of blood, measurement light having a wavelength of 450 to 500 nm, B light, G light, and an R light are sequentially taken out from a xenon lamp (30a). An internal body portion is imaged under irradiation with the measurement, B, G, and R light to obtain image data B1, B2, G2, and R2, respectively. A correlation memory (84) stores first and second correlations (84a, 84b), each being a correlation among the oxygen saturation level and intensity ratios between the image data B1 and G2 and between the image data R2 and G2. When a cumulative lighting time of the xenon lamp (30a) is less than a certain value, the oxygen saturation level is calculated using the first correlation (84a). When the cumulative lighting time equals or exceeds the certain value, the oxygen saturation level is calculated using the second correlation (84b).
