A valve drive system and method for an internal combustion engine and power output apparatus

A valve drive system (11A, 11B, 11C) for an internal combustion engine (1) is provided with: an electric motor (12) for generating a rotational driving force for driving a valve for an inlet or an outlet to a cylinder (2) when the internal combustion engine is mounted in order to open the valve in synchronization with movement of a piston in the internal combustion engine and to close; a transfer device (21, 31, 16A, 18B), which is capable of distinguishing between (i) a first condition to the rotational driving force to the valve (4, 5) to be transmitted by this from the electric motor, and (ii) a second condition to switch to stop an opening or closing operation of the valve to drive the valve or with a small lift amount; an evaluation device (6, 14C, 40) for judging whether the synchronization between the opening or closing operation of the valve and the piston movement is abnormal or not; and a fail-safe device (6, 16E, 18B) for changing the transmission device to the second condition, when it is judged by the judging device that the synchronization is abnormal.