A cap for being mounted to a vial that has a mouth located on a top side and having an opening closed by a stopper, the cap comprising: a cap body comprising a tubular body capable of being externally fitted to the mouth of the vial, and a top plate that is arranged on the top side of the tubular body and that has a through hole through which a top side surface of the stopper is exposed in a state where the tubular body is externally fitted to the mouth of the vial; a protective cover detachably attached to the top side of the top plate; and an attaching device for attaching the protective cover and the top plate to each other, wherein the top plate comprises a top plate annular portion arranged around the through hole, the protective cover comprises a cover body having a lower surface arranged away from the top plate in a state where the protective cover is attached to the top plate, and the attaching device comprises: a cover attaching portion formed into an annular shape around the through hole and projecting from the lower surface of the cover body toward the top side of the top plate annular portion or from the top side of the top plate annular portion toward the lower surface of the cover body to attach the protective cover and the top plate to each other; characterized in that the attaching device further comprises a plurality of cover projecting portions each arranged radially outside the cover attaching portion and extending radially outward from the cover attaching portion, wherein the plurality of cover projecting portions are arranged at equal intervals from each other in a circumferential direction of the cover attaching portion. a cap body comprising a tubular body capable of being externally fitted to the mouth of the vial, and a top plate that is arranged on the top side of the tubular body and that has a through hole through which a top side surface of the stopper is exposed in a state where the tubular body is externally fitted to the mouth of the vial; a protective cover detachably attached to the top side of the top plate; and an attaching device for attaching the protective cover and the top plate to each other, wherein the top plate comprises a top plate annular portion arranged around the through hole, the protective cover comprises a cover body having a lower surface arranged away from the top plate in a state where the protective cover is attached to the top plate, and the attaching device comprises: a cover attaching portion formed into an annular shape around the through hole and projecting from the lower surface of the cover body toward the top side of the top plate annular portion or from the top side of the top plate annular portion toward the lower surface of the cover body to attach the protective cover and the top plate to each other; characterized in that the attaching device further comprises a plurality of cover projecting portions each arranged radially outside the cover attaching portion and extending radially outward from the cover attaching portion, wherein the plurality of cover projecting portions are arranged at equal intervals from each other in a circumferential direction of the cover attaching portion.