With a variable valve timing system, engine operability may be lost when performing open control of lift characteristic amounts to the minimum or the maximum of a variable range when a sensor error occurs. The present invention provides a lift operation angle changing mechanism that can continuously change intake valve lift amount and operation angle within a prescribed variable range. A target operation angle is calculated based on the number of engine revolutions and the accelerator pedal opening APO. An error is determined for the operation angle sensor that detects the actual operation angle. A normal control amount is calculated based on the target operation angle and the actual operation angle, and a failure control amount is calculated based on the number of engine revolutions. Therefore, an intermediate value within the variable range can be obtained as the failure control amount. The normal control amount is output to actuator 51 under normal operation, and the failure control amount is output to actuator 51 when an error occurs.