An adaptor and coupling structure of a vehicular wiper arm are provided. The adaptor has an altered pin coupling structure that is capable of coupling to a variety of wiper arms without using rivets, is easily coupled without manipulating the pin of a wiper arm, is coupled to pivot centrally around a wiper frame, and increases the pivoting supporting strength of a wiper. The pin coupling structure includes a pivoting portion pivotably inserted in a mounting bracket of the wiper frame. The pivoting portion forms a hollow part that guides and couples with the pin of the wiper arm. The wiper frame forms a guide notch for guiding and coupling with the pivoting portion.