Abstract: A diffusing plate includes a plate body including a rectangular hole-configuring region, a plurality of holes arranged in the rectangular hole-configuring region and arranged concentrically to form a first to an N-th rectangular patterns from an inside to an outside sequentially. Scales of the first to the N-th rectangular patterns are incrementally increased, and N is a positive integer. One portion of the holes are located in an area near a center of the rectangular hole-configuring region, another portion of the holes are located in four corner areas of the rectangular hole-configuring region, each of the holes has a diameter, and the diameter of the one portion of the holes is smaller than the diameter of the another portion of the holes.