To facilitate the centering of a valve element based on a valve stem during the mounting of the valve element on the valve stem and to prevent the leak of exhaust gas.

This valve device is provided with an exhaust brake valve 16 consisting of a valve stem 20 rotatably supporting the bearing part of a valve housing 17 and a butterfly valve element 21 fixed on the valve stem 20. The valve element 21 comprises a trapezoidal part 50 molded by using a plate-form material through bending; and semicylindrical parts 51 and 51 are arranged on the extension of the two ends of the trapezoidal part 50. The valve stem 20 is provided with a columnar engaging part 41 engaged with the trapezoidal part 50 and the semicylindrical part 51; and two flanges 42 and 42 arranged at the two ends of the engaging part 41 and axially positioning the valve element 21. Contact points between the outer periphery of the engaging part 41 and the inner surface of the trapezoidal part 50 are situated in three spots at intervals in a peripheral direction.