To decrease inputting driving energy and also to achieve miniaturization by applying operating force by releasing energisation, and providing a stress concentration decreasing part on a part of a disc spring structure.

One end of an input spring 13 is connected to one end of an input spring energizing lever 12, the other end of the input spring 1 is connected to a frame 1, a shut-off spring 30 is connected to one end of a shut-off spring energizing lever 19, the other end of the shut-off spring 30 is fixed to the frame 1, a coil spring is used for the input spring 13, and the disc spring is used for the shut-off spring 30. The shut-off spring 30 is constituted of a disc spring line 31, a spring supporting member 32, and a connecting flange 33, disc springs 31a to 31e in the disc spring line 31 are so laminated that the mutual outer peripheral ends projecting in the axial direction and the mutual inner peripheral ends may be adjacent, when the plate thickness is taken as (h) and the height difference between the outside diameter side and the inside diameter side in no loading is taken as H, the value of H/h is taken as 0.5 or more. Therefore, inputting driving energy can be decreased by making use of the nonlinear characteristics of the disc springs, miniaturization is achieved, and reliability can be improved.