An image reading device includes a lens array including lenses arranged in a main scanning direction and forming an erect unmagnified optical system to converge light from a reading target, a holder holding the lens array, a light receiver to receive the light converged by the lenses, and slit units. Each slit unit includes three or more light shielding plates each separating an optical path of one lens from an optical path of another lens between the lens array and the light receiver, and side plates disposed on two sides of the light shielding plates in a direction intersecting the main scanning direction and connecting two adjacent light shielding plates. The slit units are arranged in the main scanning direction, and each two adjacent slit units are arranged with a space therebetween for one optical path of one lens.