A rod lens array to be used for an optical printer comprises a plurality of rod lenses arranged densely in array, an opaque substance filled in the gaps separating the plurality of rod lenses, and a pair of side panels sandwiching the plurality of rod lenses and the opaque substance. The rod lens array satisfies the requirement as defined by the formula: S where P is the resolution of the optical printer expressed in dpi, Z is the length of the rod lenses expressed in mm, L is the conjugate length of the rod lenses expressed in mm, and S is the flatness of the surfaces of the side panels bearing the rod lenses.