In a system for driving a three-phase rotating electrical machine of drive sources for applying separate drive forces to a rotary shaft, a phase of the rotating electrical machine, a current of which is detected by a current sensor and capable of being used for control of the rotating electrical machine, is defined as an effective sensor-phase of the rotating electrical machine. When the number of the effective sensor-phases is one, the rotating electrical machine is driven in a one-phase control mode based on the current of the effective sensor-phase under a condition where a rotation speed of the rotating electrical machine is greater than a predetermined positive threshold, and driven by the drive forces of the other drive sources under a condition where the rotation speed is not greater than the threshold.