1. An endoscope system comprising: a lighting section for applying illumination light to an object, broad band light in a visible range and narrow band light having a wavelength range in which oxyhemoglobin and deoxyhemoglobin have different absorption coefficients being used as said illumination light; an image information obtaining section for imaging said object under irradiation with said broad band light to obtain first image information, and imaging said object under irradiation with said narrow band light to obtain second image information; a normal image producing section for producing a normal image having a wavelength component of said broad band light based on said first image information; an oxygen saturation level calculator for calculating an oxygen saturation level of blood based on said second image information; a region determiner for determining in said normal image a hyperoxic region having said oxygen saturation level of a predetermined value or more and a hypoxic region having said oxygen saturation level of less than said predetermined value; and a region processor for applying a different type of process to each of said hyperoxic region and said hypoxic region, and producing an oxygen saturation image, wherein said region processor applies a color balance process to said hyperoxic region to adjust color balance, and applies a gain process to said hypoxic region to bring about color variations in said hypoxic region in accordance with difference in said oxygen saturation level, wherein said region processor applies said color balance process to said hyperoxic region without applying said gain process to said hyperoxic region, and applies said gain process to said hypoxic region without applying said color balance process to said hypoxic region, wherein said region processor applies a different type of color enhancement process to each of said hyperoxic region and said hypoxic region by applying a different tone curve to each of said hyperoxic region and said hypoxic region, and wherein said region processor applies said color enhancement process to one or both of said hyperoxic region and said hypoxic region according to an input from an input device.