To provide an opening and closing valve in an exhaust system for an internal combustion engine, remarkably reducing the size and weight of a housing.

This opening and closing valve 6 in the exhaust system for the internal combustion engine includes: the housing 61 having exhaust pipe connection portions 61b, 61c respectively provided at both opening ends of a valve opening 61a; a valve element 62 opening/closing the valve opening 61a from an upstream side in an exhaust gas flowing direction; a rotating arm 63 rotatably supporting the valve element 62; and a rotation operating shaft 64 rotatively operating the rotating arm 63. The valve element 62 opens or closes the valve opening 61a by rotatively operating the rotating arm 63 by the rotation operating shaft 64. A seating surface 61d abutting on the valve element 62 when the opening and closing valve 6 is closed is provided on the side of the upstream opening end of the valve opening 61a in the exhaust gas flowing direction so that when the opening and closing valve 6 is opened, the valve element 62 is exposed to the outside of the housing 61 from the upstream opening end of the valve opening 61a in the exhaust gas flowing direction.