1. An endoscope system comprising: an image information obtaining section for obtaining image information by imaging an observation object; a biological function information calculating section for calculating biological function information of said observation object based on said image information; a first image generating section for producing a biological function information image that images said biological function information with artificial colors, said biological function information image composed of luminance information and color difference information; an emphasized image generating section for producing an emphasized image, in said emphasized image, a low reliability area which represents a calculation result of said biological function information calculating section has low reliability in said biological function information image because of too large luminance being further brightened for emphasis without changing color of said biological function information image, said emphasized image indicating reliability of said biological function information by brightness; a second image generating section for producing based on said image information a normal light image that images said observation object irradiated with white light, wherein said emphasized image generating section produces said emphasized image by superimposing luminance of said normal light image on said biological function information image; and a display section for displaying said emphasized image, wherein said image information includes first image information, and second image information, said first image information being obtained by imaging said observation object under irradiation with first illumination light having a first wavelength range in which a light absorption coefficient varies with a variation in said oxygen saturation level, said second image information being obtained by imaging said observation object under irradiation with white light, wherein said biological function information calculating section calculates said biological function information based on said first image information and said second image information, and wherein said luminance information of said biological function information image is obtained from said second image information, and said color difference information of said biological function information image is obtained by inputting said biological function information into a color table previously stored in a memory.