To maintain good demountability and mountability of a steam port and to keep good utility thereof.

When an upper member 52 is rotated and engaged with a lower member 51, a protrusion 79 of an outer convex part 71 gets over a protrusion of an inner convex part on the way and the completion of clamping can be confirmed by a click feeling. Further, even if the clamping of the lower member 51 and the upper member 52 becomes looser, the protrusion of the inner convex part is caught by the protrusion 79 of the outer convex part 71, and the upper member 52 is not separated. This effect of preventing the looseness is due to the protrusion 79 disposed in the vertical direction, and even if the size in the horizontal direction of a steam port 41 is varied, the engagement of the lower member 51 and the upper member 52 is not obstructed. Further, the engagement is not at all obstructed by variation of the size in the height direction of the steam port 41 in some degree. Consequently, even if the engaging size of the lower member 51 and the upper member 52 is varied in some degree, the demountability and mountability of the steam port 41 are not spoiled.