A pivot cam mechanism included in a valve operating system is configured such that a coupling pin is supported at a pivot member in a position closer to a camshaft than a control shaft, and the pivot member and the driven member are integrally pivotable according to the rotation of the drive cam while changing relative attitudes of the driven member and the pivot member. Positions and shapes of the drive cam, the driven member, and the pivot member are designed so that a valve maximum acceleration point at which an acceleration of a valve body is at a maximum is located in a front part of a valve acceleration period in which the acceleration of the valve body has a positive value while the drive cam is rotating once.