A focus detection system includes a line sensor array having areas including pixels, wherein the line sensor array photoelectrically converts object light received by each pixel into electric charges, and outputs accumulated electric charges as an image signal; monitor sensor arrays, adjacent to the areas of the line sensor array, for monitoring amounts of light received thereby to output monitor signals, respectively; a controller which terminates an integral operation of the line sensor array immediately after the monitor signals reach an integration termination value; and a saturation detecting device for detecting whether the image signal reaches a predetermined level of saturation. If the saturation detecting device detects that the image signal reaches the predetermined level of saturation, the controller changes the integration termination value so that the monitor signals reach the integration termination value quicker, and makes the line sensor array perform the integral operation again.