The conventional lens array of an erecting unit magnification system is composed of an inside lens of spherical shape and an outside lens of spherical or aspherical shape, and has a problem to be improved for the resulting MTF performance. A lens array of an erecting unit magnification system is provided by stacking a first and a second planar-shaped lens array plates. Each of the first and second planar-shaped lens array plates includes a plurality of outside lenses (L1 and L4), which are regularly arranged on one side thereof, and a plurality of inside lenses (L2 and L3), which are regularly arranged on the other side thereof. Especially, the outside lenses (L1 and L4) and the inside lenses (L2 and L3) are formed based on the defining method according to the present invention.