An endoscope system comprising: a lighting means (13, 43, 48, 22) for applying normal illumination light and special illumination light to a body portion, said special illumination light having a wavelength at which oxyhemoglobin and deoxyhemoglobin have different absorption coefficients; an imaging means (47, 49) including first and second image sensors (53, 60), said second image sensor having a higher sensitivity than said first image sensor, said imaging means (47, 49) producing a special signal from said captured special illumination light reflected from said body portion; a light amount evaluation means adapted to measure a reflected light amount of said special illumination light from said body portion based on a value of weighted pixel values outputted from pixels of said first image sensor or said second image sensor; and a special image processing means adapted to measure an oxygen saturation level of blood; characterized in that the light amount evaluation means is adapted to judge whether or not said reflected light amount is lower than a predetermined sensor selection threshold value, wherein only said second image sensor is used for imaging said body portion irradiated with said special illumination light and said first image sensor is used for imaging said body portion irradiated with said normal illumination light when said reflected light amount is judged to be lower than the predetermined sensor selection threshold value, and wherein said first image sensor is used for imaging said body portion irradiated with said normal illumination light and said special illumination light when said reflected light amount is judged not to be lower than the predetermined sensor selection threshold value; a sensitizing means is adapted to sensitize said special signal output in case of using said second image sensor when said light amount evaluation means judges that said reflected light amount is lower than a predetermined start threshold value; and a special image processing means is adapted to measure an oxygen saturation level of blood based on said special signal from said second image sensor that is sensitized or not sensitized or said special signal from said first image sensor in accordance with a judgment result of said light amount evaluation means, and producing a special image depicting said oxygen saturation level. a lighting means (13, 43, 48, 22) for applying normal illumination light and special illumination light to a body portion, said special illumination light having a wavelength at which oxyhemoglobin and deoxyhemoglobin have different absorption coefficients; an imaging means (47, 49) including first and second image sensors (53, 60), said second image sensor having a higher sensitivity than said first image sensor, said imaging means (47, 49) producing a special signal from said captured special illumination light reflected from said body portion; a light amount evaluation means adapted to measure a reflected light amount of said special illumination light from said body portion based on a value of weighted pixel values outputted from pixels of said first image sensor or said second image sensor; and a special image processing means adapted to measure an oxygen saturation level of blood; characterized in that the light amount evaluation means is adapted to judge whether or not said reflected light amount is lower than a predetermined sensor selection threshold value, wherein only said second image sensor is used for imaging said body portion irradiated with said special illumination light and said first image sensor is used for imaging said body portion irradiated with said normal illumination light when said reflected light amount is judged to be lower than the predetermined sensor selection threshold value, and wherein said first image sensor is used for imaging said body portion irradiated with said normal illumination light and said special illumination light when said reflected light amount is judged not to be lower than the predetermined sensor selection threshold value; a sensitizing means is adapted to sensitize said special signal output in case of using said second image sensor when said light amount evaluation means judges that said reflected light amount is lower than a predetermined start threshold value; and a special image processing means is adapted to measure an oxygen saturation level of blood based on said special signal from said second image sensor that is sensitized or not sensitized or said special signal from said first image sensor in accordance with a judgment result of said light amount evaluation means, and producing a special image depicting said oxygen saturation level.