An apparatus that detects an in-focus state according to the present invention has a plurality of line sensors arranged on a projection area of an optical image-forming system; a plurality of monitor sensors arranged on the projection area with each monitor sensor positioned adjacent to a corresponding line sensor and monitoring a quantity of light incident on the corresponding line sensor; an electric charge accumulation controller that controls the timing of completion of the accumulation of electric charges in the plurality of line sensors on the basis of monitor signals output from corresponding monitor sensors; that controls the timing of the commencement of the subsequent accumulation of electric charges in the plurality of line sensors; and a signal output processor that reads the accumulated electric charges from the plurality of line sensors and then outputs image signals corresponding to an object image. The electric charge accumulation controller commences accumulation of electric charges before the reading of all electric charges in the plurality of line sensors finishes.