A valve operation mode changing system for the intake or exhaust valves of an internal combustion engine is controlled in response to engine speed and temperature of the working oil utilized in the system in such a way as to permit the valve operating mode to be changed from a low-speed to a high-speed mode in response to engine speed requirements only when the temperature of the working oil exceeds a predetermined value. The system also provides for terminating fuel supply to the engine when the temperature of the working oil is below a predetermined temperature and the speed of the engine exceeds a predetermined temperature.