1. An electronic endoscope system comprising: a light source device for sequentially emitting plural kinds of light having different wavelength bands from each other; an electronic endoscope for sequentially illuminating the plural kinds of light emitted from said light source device to a subject tissue containing blood vessels inside a body cavity, sequentially receiving the plural kinds of reflected light of the illuminated light from the subject tissue, and sequentially outputting image data of acquired images corresponding to the plural kinds of received light having the different wavelength bands; a blood vessel characteristics amount setting unit configured to set as a blood vessel characteristics amount related to the blood vessel a diameter of the blood vessel from the image data of at least one of the acquired image outputted from said electronic endoscope; an interest region setting unit configured to set a region of interest in the acquired image by said electronic endoscope based on the blood vessel characteristics amount calculated by said blood vessel characteristics amount setting unit; a first image production unit configured to produce a first oxygen saturation level image representing a distribution of an oxygen saturation level in the blood vessel in simulated color from the image data of the acquired images; a second image production unit configured to produce a second oxygen saturation level image in which the oxygen saturation level within the region of interest set by said interest region setting unit is selectively enhanced in the first oxygen saturation level image produced by said first image production unit; and an image display unit configured to display in the simulated color both the first oxygen saturation level image produced by said first image production unit and the second oxygen saturation level image produced by said second image production unit, wherein said blood vessel characteristics amount setting unit applies a two-dimensional filter corresponding to the diameter of the blood vessel to the image data of the acquired image to extract a blood vessel having the diameter of the blood vessel, and wherein said interest region setting unit sets a region of the extracted blood vessel by said blood vessel characteristics amount setting unit as the region of interest.