An image reading device can easily increase or improve the depth of field. The image reading device includes a lens array (1) including lenses (2) arrayed in a main scanning direction, a sensor array (4) including sensor elements (4) arrayed in the main scanning direction to receive light converged by the lenses (2), and an overlap preventer located between the lens array (1) and the sensor array (4) to prevent an overlap of images formed by the lenses (2). The overlap preventer includes a slit assembly (5) including at least one specific-light blocker or optically transparent columns (13).