An endoscope system comprising: a lighting means adapted to sequentially and repeatedly apply a plurality of types of illumination light having different wavelength bands to an internal body portion; an image sensor adapted to sequentially capture reflected light from said internal body portion, and to produce a plurality of types of frame images corresponding to said types of illumination light; a video image processing means adapted to produce a plurality of types of video images based on said plurality of types of frame images by controlling emission timing of the lighting means; a brightness detection means adapted to detect brightness of a specific type of frame image out of said plurality of types of frame images; an exposure condition determination means (76, 77) adapted to determine based on said detected brightness, exposure conditions of said plurality of types of frame images to be obtained; an exposure control means adapted to regulate exposure of said image sensor in accordance with said determined exposure conditions, characterized in that said lighting means is adapted to apply normal light which is white light and special light to the internal body portion; said video image processing means is adapted to produce, in a normal observation mode, a normal video image based on a frame image obtained under said normal light, and in a special observation mode, an oxygen saturation frame image for producing an oxygen saturation video image, a normal frame image for producing a normal video image and a vessel pattern frame image for producing a vessel pattern video image are obtained by the video image processing means in a sequential manner; wherein in the special observation mode said brightness detection means is adapted to detect brightness of a key video image chosen from the three types of video images, wherein the brightness detection means detects the brightness of an entire latest frame image of the key video image and said exposure condition determination means (76, 77) sets the exposure condition of the image sensor to control the brightness of all the three types of video images based on the brightness of the latest frame image of the key video image. a lighting means adapted to sequentially and repeatedly apply a plurality of types of illumination light having different wavelength bands to an internal body portion; an image sensor adapted to sequentially capture reflected light from said internal body portion, and to produce a plurality of types of frame images corresponding to said types of illumination light; a video image processing means adapted to produce a plurality of types of video images based on said plurality of types of frame images by controlling emission timing of the lighting means; a brightness detection means adapted to detect brightness of a specific type of frame image out of said plurality of types of frame images; an exposure condition determination means (76, 77) adapted to determine based on said detected brightness, exposure conditions of said plurality of types of frame images to be obtained; an exposure control means adapted to regulate exposure of said image sensor in accordance with said determined exposure conditions,