An endoscope system (10, 500, 600, 700) comprising: a processor device adapted to generate a spectral estimation image from a normal observation image to calculate a first oxygen saturation from the spectral estimation image, and to generate a first oxygen saturation image from the first oxygen saturation, an endoscope including an insertion portion for insertion into a subject, an operation portion provided at a base end of the insertion portion and comprising a mode selector, for switching operation between a normal observation mode and a special observation mode in which an oxygen saturation image can be obtained by imaging the oxygen saturation of blood haemoglobin of the observation target; a narrowband light source having a first blue light source for emitting a first blue laser light beam having a center wavelength of 473 nm in the special observation mode and a second blue laser light source for emitting a second blue laser light beam having a center wavelength of 445 nm a light guide wherein the first and second blue light beams are incident on the light guide, wherein the light guide is configured to cause the first and second blue laser light beams to propagate to a distal portion of the insertion portion; wherein the processor device is adapted to generate a second oxygen saturation image for display based on a second oxygen saturation; wherein the distal portion includes: an illumination optical system comprising: a phosphor for emitting fluorescence due to the first or second blue laser light beams, an illumination lens, wherein the light emitted by the phosphor is emitted to the observation target through the illumination lens, wherein the first and second blue laser light beams propagating in the light guide are incident on the phosphor from the light guide; an imaging optical system; wherein in the normal observation mode the second blue laser light beam is incident on the phosphor to emit white light having a range covering the entire visible light region; wherein in the special observation mode the first and second blue laser light beams are alternately incident on the phosphor to emit white light having two different spectra covering the entire visible light region; and a display unit for displaying the first oxygen saturation image and the second oxygen saturation image. a processor device adapted to generate a spectral estimation image from a normal observation image to calculate a first oxygen saturation from the spectral estimation image, and to generate a first oxygen saturation image from the first oxygen saturation, an endoscope including an insertion portion for insertion into a subject, an operation portion provided at a base end of the insertion portion and comprising a mode selector, for switching operation between a normal observation mode and a special observation mode in which an oxygen saturation image can be obtained by imaging the oxygen saturation of blood haemoglobin of the observation target; a narrowband light source having a first blue light source for emitting a first blue laser light beam having a center wavelength of 473 nm in the special observation mode and a second blue laser light source for emitting a second blue laser light beam having a center wavelength of 445 nm a light guide wherein the first and second blue light beams are incident on the light guide, wherein the light guide is configured to cause the first and second blue laser light beams to propagate to a distal portion of the insertion portion; wherein the processor device is adapted to generate a second oxygen saturation image for display based on a second oxygen saturation; wherein the distal portion includes: an illumination optical system comprising: a phosphor for emitting fluorescence due to the first or second blue laser light beams, an illumination lens, wherein the light emitted by the phosphor is emitted to the observation target through the illumination lens, wherein the first and second blue laser light beams propagating in the light guide are incident on the phosphor from the light guide; an imaging optical system; wherein in the normal observation mode the second blue laser light beam is incident on the phosphor to emit white light having a range covering the entire visible light region; wherein in the special observation mode the first and second blue laser light beams are alternately incident on the phosphor to emit white light having two different spectra covering the entire visible light region; and a display unit for displaying the first oxygen saturation image and the second oxygen saturation image. a phosphor for emitting fluorescence due to the first or second blue laser light beams, an illumination lens, wherein the light emitted by the phosphor is emitted to the observation target through the illumination lens, wherein the first and second blue laser light beams propagating in the light guide are incident on the phosphor from the light guide; an imaging optical system; wherein in the normal observation mode the second blue laser light beam is incident on the phosphor to emit white light having a range covering the entire visible light region; wherein in the special observation mode the first and second blue laser light beams are alternately incident on the phosphor to emit white light having two different spectra covering the entire visible light region; and a display unit for displaying the first oxygen saturation image and the second oxygen saturation image.