A control apparatus controls rotating electrical machine which is applied to a vehicle in which an engine is automatically stopped in the case where predetermined automatic stop conditions are satisfied, and, the engine is automatically restarted in the case where predetermined restarting conditions are satisfied, the rotating electrical machine receiving supply of an exciting current from a transistor-chopper type exciting circuit in which a first pair of facing arms of a bridge circuit is configured with power transistors, and a second pair of arms is configured with diodes, and the rotating electrical machine having a power generation function based on rotational force of the engine. The control apparatus executes first grounding control in which, during automatic stop of the engine, among the first pair of arms, the power transistor connected on an earth side of the rotating electrical machine is put into an ON state.