An endoscope system has a signal processing circuit for converting an image signal output of a solid state imaging device into a standard image signal, a monitor for providing a display of the standard image signal, an automatic gain control circuit for outputting an image signal whose level is automatically adjusted in accordance with the level of the image signal; and a gain variation region setting circuit for outputting a control signal to set the gain variation region of the automatic gain control circuit. With this arrangement, it is possible to properly set the AGC variation region of the AGC of the signal processing apparatus in accordance with such factors as the number of pixels of the solid state imaging device which constitutes a part of an electronic endoscope connected.