An electronic endoscope system is composed of an electronic endoscope, a light source apparatus, and a temperature converter. The electronic endoscope has a CMOS sensor in a distal portion of an insert section to be inserted into a patients body cavity. Illumination light from the light source apparatus is applied to the body cavity through the distal portion. The temperature converter obtains an average pixel value of an optical black (OB) region out of an imaging signal from the CMOS sensor, and converts the average OB pixel value into a temperature of the CMOS sensor on a frame-by-frame basis with the use of data in a temperature conversion table. The table represents a relationship between the average OB pixel value and the temperature of the CMOS sensor. Light quantity of the illumination light is adjusted in accordance with the temperature of the CMOS sensor to prevent deterioration of image quality.
