In a variable lift valve operating system for an internal combustion engine, an actuator for rotatably driving a control shaft of a variable lift mechanism is mounted on an engine body. The actuator includes an electric motor, a deceleration mechanism, a transmission mechanism interposed between the control shaft and the deceleration mechanism, and a default mechanism. A deceleration mechanism accommodation part for accommodating the deceleration mechanism and a default mechanism accommodation part for accommodating the default mechanism are formed in a casing of the actuator so as to sandwich therebetween a thermally vulnerable part which is directly connected to the control shaft. Therefore, it is possible to enhance the degree of freedom of amounting position of the actuator while heat damage is prevented from generating in the thermally vulnerable part of the actuator.