1. An endoscope system comprising: an illumination device for projecting illumination light toward a subject; an image capturing device for capturing an image of light reflected from said subject; a base image generator for generating a base image based on an imaging signal obtained by the image capturing device; an extractor for extracting first-layer blood vessels located at a particular depth from said imaging signal so as to generate a first-layer-blood-vessel extracted image and extracting second-layer blood vessels located at a depth deeper than said first-layer blood vessels from said imaging signal so as to generate a second-layer-blood-vessel extracted image; a blood vessel emphasized/de-emphasized image generator for generating a blood vessel emphasized/de-emphasized image, in which said first-layer blood vessels or said second-layer blood vessels are emphasized or de-emphasized, by adding said first-layer-blood-vessel extracted image or said second-layer-blood-vessel extracted image to said base image or subtracting said first-layer-blood-vessel extracted image or said second-layer-blood-vessel extracted image from said base image; and a displaying device for displaying said blood vessel emphasized/de-emphasized image, wherein said image capturing device captures an image of said subject by a color imaging element, wherein said extractor includes: a B/G image generating section for generating a B/G image based on a luminance ratio between a blue signal outputted from a blue pixel of said imaging element and a green signal outputted from a green pixel of said imaging element, wherein a magnitude relation of luminance of said first-layer blood vessels to luminance of mucous membrane and to luminance of middle-deep blood vessels is set such that the luminance of said first-layer blood vessels is less than the luminance of mucous membrane which is less than the luminance of middle-deep blood vessels and the magnitude relation is established in said B/G image; and a blood vessel extracting section for extracting said first-layer blood vessels located at a particular depth from said B/G image so as to generate said first-layer-blood-vessel extracted image and extracting said second-layer blood vessels located at a depth deeper than said first-layer blood vessels from said B/G image so as to generate said second-layer-blood-vessel extracted image, and wherein said blood vessel extracting section extracts a first frequency component containing said first-layer blood vessels from said B/G image so as to generate said first-layer-blood-vessel extracted image in which the luminance of said first-layer blood vessels is negative and the luminance of said mucous membrane is zero, or extracts a second frequency component containing said second-layer blood vessels from said B/G image so as to generate said second-layer-blood-vessel extracted image in which the luminance of said first-layer blood vessels is positive and the luminance of said mucous membrane is zero.