A rotation axis O2 of a shaft 3 is attached thereto with double-offset in the X and Y directions with respect to a center line O1 of a valve seat 2. A bevel process is implemented on the outer peripheral surface of a valve 1 such that the valve 1 and the valve seat 2 is line-sealed, and the sign of the angles of inclination is switched between plus and minus at two places of the outer peripheral surface of the valve 1 deviated from the center of the axis. Further, a section where the angle of inclination is constant is prepared in each of the sections connecting the two places, while the other sections are smoothly connected to each other.