To provide a valve element assembling method for an exhaust system valve and the exhaust system valve which stably provides a high gas sealing performance in closing a valve irrelevant to the accuracy of a shaft hole position for inserting a supporting shaft of the valve element.

A valve seal face is constituted by the contact between a seal spherical face 11 formed in a valve element 1 and a seal taper face 21 formed in a valve seat 2, a shaft hole 4 having a diameter which is approximately coincided with the diameter of a supporting shaft 3 is bored in a first shaft support part 1c of the valve element 1, an oblong hole 5 whose diameter in the pipe shaft direction is approximately coincided with the diameter of the supporting shaft 3 and diameter in the pipe shaft perpendicular direction is longer than the diameter of the supporting shaft 3 is bored in a second support part 2c of the valve seat 2, the valve element 1 is set in the valve seat 2, the supporting shaft 3 is inserted into the oblong hole 5 and the shaft hole 4, and the both end parts of the supporting shaft 3 are welded and fixed to the oblong hole 5 of the second shaft support part 2c of the valve seat 2 while holding the close seal face contact between the valve element 1 and the valve seat 2.