An endoscope system has a light source system configured to emit illumination light on a target area, a scanner configured to periodically scan the illumination light over a target area at predetermined time intervals, and an imager configured to receive the illumination light reflected from the target area and to acquire in succession a sequence of image-pixel signals. Further, the endoscope system has a luminance detector that detects in succession a sequence of luminance data of the object image from the sequence of image-pixel signals; and a brightness adjuster that adjusts the brightness of an observed image on the basis of the sequence of luminance data. Then, the brightness adjuster adjusts an amount of illumination light in accordance with a scanning position of the illumination light.