1. An electronic endoscope system comprising: an emission section for emitting white broadband light and narrow band light simultaneously or sequentially to illuminate a tissue site in a body cavity, the tissue site including a blood vessel, the broadband light having a wavelength range from blue to red, the narrow band light being in a specific wavelength range; an imaging section for capturing an image of the tissue site during the illumination and outputting image signals in time order, the imaging section having a blue pixel, a green pixel, and a red pixel arranged therein, the blue pixel sensing blue light to generate a blue color signal, the green pixel sensing green light to generate a green color signal, the red pixel sensing red light to generate a red color signal; a data obtaining section for obtaining broadband image data and narrow band image data or mixed image data based on the image signals, the broadband image data corresponding to the image capture under the broadband light, the narrow band image data corresponding to the image capture under the narrow band light, and the mixed image data being a combination of the broadband image data and the narrow band image data; a brightness ratio calculator for calculating a brightness ratio between the broadband image data and the narrow band image data or a brightness ratio between the blue color signal and the green color signal in the mixed image data on a picture element basis; a depth correlation information storage for storing depth correlation information between the brightness ratio and a depth of the blood vessel; a depth determining section for referring to the depth correlation information to determine whether the picture element includes the blood vessel and the depth of the blood vessel on the picture element basis with the use of the calculated brightness ratio; a blood vessel region extractor for extracting a specific blood vessel region based on the determined depth of the blood vessel; and an image processing section for performing image processing to the specific blood vessel region or a region other than the specific blood vessel region within a broadband image based on the broadband image data.