To provide a prescribed opening area of a slit of an exhaust valve element by providing a rib across the whole circumference in the internal circumferential side of an end face opposed to a presser face of a presser member for the exhaust valve element.

A plane 4 opposed to a presser face 12 of a presser member 10 of a rib 3 is perpendicular to the axial direction of an exhaust valve element 1, the outer circumferential face 5 of the rib 3 is perpendicular to the plane 4, and a plane 6 having no rib of the end face 7 opposite to the presser face 12 of the presser member 10 of the exhaust valve element 1 is set perpendicular to the axial direction of the exhaust valve element 1. Supposing that a radial dimension of the plane 4 perpendicular to the axial direction of the exhaust valve element 1 of the rib 3 is set to (c) and a stage difference dimension in a staged side of the rib 3 is set to (a), a dimension ratio is a/c≤1. This constitution prevent the rib 3 from buckling, making constant the pushing direction of the exhaust valve element 1, stably fix the exhaust valve element 1, and facilitates in obtaining a prescribed opening area of the slit 12 of the exhaust valve element 1.