An automotive supplementary sun visor to shield a windshield portion around a rearview mirror between the right and left sun visors using a slit from an upper side edge of the shade plate body toward a center portion and structuring the shade plate body detachable in an attaching support shaft portion of a rearview mirror by the slit and its own flexibility. The present invention does not require a design change to an existing sun visor or rearview mirror within a passenger room or a specification change of a body construction of a motor vehicle, can be attached to a support shaft portion of the existing rearview mirror by utilizing the flexibility of the shade plate itself, and securely dims or blocks sunlight from a periphery of the rearview mirror not otherwise accounted for by existing sun visors.