A focus detection system includes a line sensor array wherein the line sensor array photoelectrically converts object light into an electric charge, and outputs the accumulated electric charge as an image signal; a monitor sensor array for monitoring an integral value of the line sensor array to output a monitor signal; a controller; and an amplifier. The controller detects the integral value of the line sensor array from the monitor signal, continues to make the line sensor array perform an integral operation until a secondary maximum integral time elapses if, upon a lapse of a predetermined primary maximum integral time, a value of the monitor signal needs to be amplified at an amplification factor greater than a maximum amplification factor of the amplifier, and increases the amplification factor of the amplifier after a lapse of the secondary maximum integral time.