In an engine valve operating system, a camshaft (36) is supported via a bearing (41) on a cylinder head (5) having an intake valve (29i) and an exhaust valve (29e)provided therein, and an intake rocker arm (38i)and an exhaust rocker arm (38e)are mounted on a rocker arm shaft (42) supported on the cylinder head (5) so as to be parallel to the camshaft (36), the intake rocker arm (38i)and the exhaust rocker arm (38e)respectively providing a connection between the camshaft (36) and the intake valve (29i)and between the camshaft (36) and the exhaust valve (29e). A fixing bolt (44) that abuts against an end part of the rocker arm shaft (42) so as to restrict movement thereof in the thrust direction is screwed into the cylinder head (5), and a flange seat (44a)that abuts against one side face of the bearing (41) so as to restrict movement thereof in the thrust direction is formed integrally with the fixing bolt (44). Therefore, there is provided a compact engine valve operating system in which means for restricting movement in the thrust direction of a camshaft and a rocker shaft is shared to thus reduce the number of components and simplify the structure.