PURPOSE: To obtain the sealability when a seat plate is deformed by incorporating a packing of a sectional rectangular shape in a pressed state between a skirt side protrusion and a seat plate side protrusion, and pressing both the edges of the packing to the inner surface of the plate side protrusion by a repulsion force due to the bending of the packing.

CONSTITUTION: A packing 47 is contained in the recess 45 of a seat plate 37, and the center of the packing 47 is pressed by a skirt side protrusion 51. After the packing 47 is bent from its center, the caulked part 41. to be formed on the outer periphery of the plate 37 is caulked to the rear surface 43 of a skirt part 35, and the plate 37 is fixed to a tank 31. The packing 47 of a sectional rectangular shape is contained between the protrusion 51 and a seat plate side protrusion 53 in the pressed state. Thus, the packing 47 having elasticity is bent at its center, and both the edges (a), (b) of the packing 47 are rigidly pressed to the inner surface of the protrusion 53 by the repulsion force due to the bending. In this manner, an effective sealing is obtained mainly by both the edges (a), (b) of the packing 47 and the end (c) of the protrusion 51.