A mark placed on the original support plate outside the original support zone for adjusting the position of a solid-state line image sensor of a line-by-line image dissecting system including a first mark member extending in a direction perpendicular to the auxiliary scanning direction of an original, and a second mark member parallel to the first mark member and spaced apart therefrom by a spacing interval substantially equal to the width of an array of light receiving elements of the solid-state line image sensor when projected on the surface of the sensor as an image of a reduced scale. The first mark member includes a solid straight line or a broken straight line, and the second mark member includes one or a plurality of broken straight lines parallel to one another. Dashes constituting each broken straight line are spaced apart from one another by a spacing interval which differs from one broken straight line to another, to enable different broken straight lines to be distinguished from one another. The mark further includes a plurality of bars or bar elements extending to be projected on the surface of the sensor as bar images or bar element images between the limits of the initial setting tolerance of the solid-state line image sensor.