An endoscope system includes an illumination apparatus (14, 104) for applying imaging light to body tissue in a tube of a body cavity. An electronic endoscope (12) images the body tissue illuminated with the imaging light, and outputs a blue image signal (SB) of blue and a green image signal (SG) of green. A signal processing device detects specific body tissue (such as blood vessels) in the body tissue according to the blue and green image signals. A calibration device (52, 60) refers to a reference ratio (SB/SG) predetermined according to a reflectivity of the body tissue in relation to the blue and green, and calibrates the blue and green image signals to set a ratio between the blue and green image signals equal to the reference ratio.
