The present invention relates to a polygonal rib structure having ribs directed orthogonal to one plane and intersecting each other to form polygonal shapes. More particularly, to a shock absorbing polygonal rib structure. In a polygonal rib structure having ribs directed orthogonal to one plane and intersecting each other to form polygonal shapes, a thickened central portion located at a substantially central portion in the longitudinal direction of each side of the polygonal rib is bisected. The opposed parting surfaces formed upon the bisection are spaced from each other to form an opening portion, and each side of the polygonal rib is so shaped that the cross-sectional area of the opening portion is reduced as one goes from the opening end toward the depth. Thus, an opened polygonal rib structure is formed.