Focus detection device

A focus detection device, comprising:

• a plurality of line sensors, each having a plurality of pixels, wherein each of said line sensors photoelectrically converts and integrates light from an object received by each of the pixels so as to output the received light as an image signal;
• a plurality of monitor sensors provided adjacent to said line sensors, respectively, each of said monitor sensors monitoring an integrated value of said adjacent line sensor;
• a control device which amplifies an output of each of the monitor sensors and outputs the amplified output as a monitor signal, and terminates integration of said line sensor corresponding to said monitor sensor when said monitor signal reaches a predetermined termination value;
• and
• a gain adjustment device which adjusts a gain level of the control device in accordance with each integration time during integration of the line sensor,
• wherein said gain adjustment device adjusts the gain level of the control device so that an appropriate integrated value of the line sensor becomes an approximately predetermined value when the monitor signal reaches the predetermined termination value regardless of the integration time.