An endoscope system (10, 120) comprising: an illuminating section for projecting illumination light to a portion to be observed, the portion including a blood vessel; an image signal obtaining section (34, 60) having an image sensor, the image signal obtaining section capturing reflection light reflected from the portion using the image sensor to obtain at least first to third image signals, the first image signal being obtained from first reflection light including a wavelength range in which an absorption coefficient varies in accordance with oxygen saturation of hemoglobin in blood, the second image signal being obtained from second reflection light including a wavelength range in which an absorption coefficient varies in accordance with blood volume, the third image signal being a reference signal of the first and second image signals; a signal ratio calculator for calculating signal ratios for the first to the third image signal and the second to the third image signal; an oxygen saturation calculator for obtaining oxygen saturation information on a picture element by picture element basis based on the signal ratios; a reliability calculator for determining reliability of the oxygen saturation information on the picture element by picture element basis in at least one of the first to third image signals; an image processing section for generating an oxygen saturation image based on color difference signals corresponding to the oxygen saturation information and the reliability, the color difference signals being obtained by referring to a color tone information memory which stores a relation between the oxygen saturation information and the color tone information, wherein the color tone information is a signal value of a color difference signal; and a display section for displaying the oxygen saturation image, wherein the image processing section further includes: an oxygen saturation image generator for correcting the color tone information with the reliability and generating the oxygen saturation image from corrected color tone information, and the color tone information is read out from the color tone information memory based on the oxygen saturation information. an illuminating section for projecting illumination light to a portion to be observed, the portion including a blood vessel; an image signal obtaining section (34, 60) having an image sensor, the image signal obtaining section capturing reflection light reflected from the portion using the image sensor to obtain at least first to third image signals, the first image signal being obtained from first reflection light including a wavelength range in which an absorption coefficient varies in accordance with oxygen saturation of hemoglobin in blood, the second image signal being obtained from second reflection light including a wavelength range in which an absorption coefficient varies in accordance with blood volume, the third image signal being a reference signal of the first and second image signals; a signal ratio calculator for calculating signal ratios for the first to the third image signal and the second to the third image signal; an oxygen saturation calculator for obtaining oxygen saturation information on a picture element by picture element basis based on the signal ratios; a reliability calculator for determining reliability of the oxygen saturation information on the picture element by picture element basis in at least one of the first to third image signals; an image processing section for generating an oxygen saturation image based on color difference signals corresponding to the oxygen saturation information and the reliability, the color difference signals being obtained by referring to a color tone information memory which stores a relation between the oxygen saturation information and the color tone information, wherein the color tone information is a signal value of a color difference signal; and a display section for displaying the oxygen saturation image, wherein the image processing section further includes: an oxygen saturation image generator for correcting the color tone information with the reliability and generating the oxygen saturation image from corrected color tone information, and the color tone information is read out from the color tone information memory based on the oxygen saturation information.