PURPOSE: To prevent continuously and through long duration the channel between a high temperature fluid and a low temperature fluid from leakage from a cool operation time to a hot operaton time by a method wherein a back plate having a suitable curvature is detachably attached on the back part of a sealing member, the regulating effect is given to the back plate so that the bending deformation of the sealing member is limited within a curvature less than the fatigue bending stress.

CONSTITUTION: A front plate 6 is detachably attached with designated inclined angle to be contacted with an upper selector plate 7 at the upper and lower parts of a radiating direction partition plate 5. An elastic thin plate sealing member 9 is detachably attached on the back part in the inverse direction of the rotating direction of the front plate 6. The back plate 10 is detachably attached on the back part of the elastic thin plate sealing member 9 so that the deformation curvature of the elastic thin plate sealing member 9 is limited within the curvature less than the fatigue bending stress when the elastic thin plate sealing member 9 is slid with the upper selector plate 7.