An electronic endoscope having a solid state image pickup device supplies an image signal which is converted into a digital signal and subject to a gamma correction in a gamma correcting portion in accordance with gamma correcting data stored in and read from a look-up table. The look-up table stores only data necessary for the gamma correction with respect to the image signal, so that data for the gamma correction is added to the image signal in the gamma correcting portion to output a gamma-corrected signal.