PURPOSE: To maintain fitting strength of a valve stem into a valve body by mounting a junction part which joints both side pieces to each other in a recessed groove in a valve stem top end surface, and forming a fitting groove which forms a complementary shape to the junction part at the peripheral edge part of the valve body.

CONSTITUTION: Inside a recessed groove 4 formed in the top end surface 3a of a valve stem 3, a part between the central parts of both side pieces 3b between which the recessed groove 4 is put and face each other is jointed, and a junction part 5 where one end separated from the top end surface 3a of the valve stem 3 is expanded and opened in an arc shape. In a part which meets the valve stem 3 at the peripheral edge part of the valve body 2, a fitting groove 6 which forms an additional shape to the junction part 5 is formed. For assembling the valve body 2 and the valve stem 3, the valve stem 3 is pressed in an arrowed direction with the fitting groove 6 and the junction part 5 aligned, and the valve body 2 is fitted in the recessed groove 4 in the valve stem 3. It is thus possible to prevent by the junction part 5 the top end parts of both side pieces 3b from being expanded and opened outwards each other and obtain great junction strength when pressing-in by making the width of the recessed groove 4 in the valve stem 3 smaller than the plate thickness of the valve body 2.