Abstract: An instrument includes a holder for holding a medium, a cap capable of covering a tip end portion of the medium, and a knock member provided to be capable of knocking to attach and detach the cap. The cap is connected to the knock member via a rotational transform mechanism. The rotational transform mechanism advances the cap in accordance with one knock of the knock member in the axial direction, the rotational transform mechanism rotates the cap around an axis parallel to the axial direction, then retreats the cap, so that the state in which the cap covers the tip end portion of the medium and the state in which the cap allows the tip end portion of the medium to be exposed are switchable for each knock of the knock member.