A shut-off instrument for a flow medium, in particular a slide valve instrument designed for pressurization on both sides with an instrument housing forming a flow channel and a slide valve chamber and with a soft-sealing shut-off element closing off the flow channel and with a slide valve stem which penetrates a seal arrangement and/or bearing arrangement in a housing neck extension delimiting the slide valve chamber and which is rotationally connected to a stem nut coupled to the shut-off element by a screw contact. The shut-off element is designed to include more than one part and at least one external dimension of the main body is slightly smaller than a nominal width of the flow channel and seal elements are arranged on opposite support surfaces of the main body via an engaged connection projecting over the external dimension with a peripheral edge collar.