A control device for controlling a rotary electrical machine functioning as a motor and as a power generator, the device being provided with a power conversion circuit serving to function as an inverter for supplying a drive current from a battery to the rotary electrical machine and as a rectifier for rectifying the power generation output of the rotary electrical machine and supplying the output to the battery; and being provided with a controller for controlling the conversion circuit so that three-phase armature coils of the rotary electrical machine are short-circuited when the battery is disconnected from the conversion circuit and the DC output voltage of the conversion circuit becomes excessive, after which, when the DC output voltage of the conversion circuit decreases to a set low voltage, the short circuit of one phase of the armature coil is released and only two phases are short-circuited.