An apparatus that detects an in-focus state has a plurality of line sensors and an image signal output processor. The plurality of line sensors is arranged on a projection area. The image signal output processor outputs image signals of an object on the basis of electric charges accumulated in the plurality of line sensors. Each line sensor is equipped with a plurality of pairs of photoelectric converters, which are arrayed along the longitudinal direction of the line sensor, and a plurality of image-pixel signal-reading circuits. Each image-pixel signal-reading circuit reads electric charges from a corresponding pair of photoelectric converters. Also, each image-pixel signal-reading circuit has a first circuit that reads electric charges accumulated in one photoelectric converter and a second circuit that reads electric charges accumulated in the other photoelectric converter. Each image-pixel signal-reading circuit is positioned between the corresponding pair of photoelectric converters and segments of the first and second circuits being shared.