An endoscope system comprising: a first semiconductor light source for emitting first illumination light having a first wavelength; a second semiconductor light source for emitting second illumination light having a second wavelength, an isosbestic wavelength of oxyhemoglobin and deoxyhemoglobin existing between said first and second wavelengths; an imaging device for imaging an internal body portion irradiated with said first or second illumination light; a vessel pattern image generating section for producing a vessel pattern image based on an image signal obtained by said imaging device including a vessel contrast judging section for judging whether or not a vessel contrast between a vessel area and an area other than said vessel area in said vessel pattern image coincides with a standard value; and a contrast adjusting section (62, 63, 65) for adjusting said vessel contrast in the case where said vessel contrast does not coincide with said standard value, characterized in that the endoscope system further comprises an oxygen saturation image generating section for producing an oxygen saturation image based on an image signal obtained by said imaging device, and said contrast adjusting section (62, 63, 65) is configured to adjust change of said vessel contrast caused by change of an oxygen saturation level of blood in said vessel pattern image, and the light intensity of said first illumination light is higher than the light intensity of said second illumination light, or the light intensity of said second illumination light is higher than the light intensity of said first illumination light. a first semiconductor light source for emitting first illumination light having a first wavelength; a second semiconductor light source for emitting second illumination light having a second wavelength, an isosbestic wavelength of oxyhemoglobin and deoxyhemoglobin existing between said first and second wavelengths; an imaging device for imaging an internal body portion irradiated with said first or second illumination light; a vessel pattern image generating section for producing a vessel pattern image based on an image signal obtained by said imaging device including a vessel contrast judging section for judging whether or not a vessel contrast between a vessel area and an area other than said vessel area in said vessel pattern image coincides with a standard value; and a contrast adjusting section (62, 63, 65) for adjusting said vessel contrast in the case where said vessel contrast does not coincide with said standard value, characterized in that the endoscope system further comprises an oxygen saturation image generating section for producing an oxygen saturation image based on an image signal obtained by said imaging device, and said contrast adjusting section (62, 63, 65) is configured to adjust change of said vessel contrast caused by change of an oxygen saturation level of blood in said vessel pattern image, and the light intensity of said first illumination light is higher than the light intensity of said second illumination light, or the light intensity of said second illumination light is higher than the light intensity of said first illumination light.