For a method fabricating a dihedral corner reflector array optical element, a molding die having a reversal shape of the prism bodies is used. Each prism body has a frustum shape having an end plane whose area is smaller than that of the base plane side of the substrate. Each prism body is composed of a rectangular parallelepiped portion including the orthogonal plane sides to be a dihedral corner reflector and a taper portion integrated therewith having sides being non-parallel to the orthogonal plane sides. The method includes clamping the molding die to define a cavity therein; forming the optical element made from molten resin in the cavity; and parting the optical element from the molding die. The molding dies are relatively moved in a parting direction that the molding die leaves the orthogonal plane sides before anything else to part the optical element.