In a housing having an inlet opening and an outlet opening, the shut-off mechanism has a rotary plug for interrupting the flow of flowable material through the housing. Via the rotary plug the outlet opening is gas-tightly sealed and a flow of material through the inlet opening is prevented. The rotary plug is provided with a sealing member, which is rotatable about an axis which is approximately at right angles to the axis of rotation of the rotary plug. The sealing member cooperates with the valve seat, which is associated with the outlet opening. A compression spring extends between the sealing member and the rotary plug and is laterally spaced from the axis of rotation of the sealing member. Upon a movement of the rotary plug the compression spring will impart a certain rotation of the sealing member without a need for a drive.