A control apparatus of a rotary machine controls drive of the rotary machine with winding groups. The control apparatus includes electric power converters in multiple systems, a failure detection portion, and a controller. The electric power converter has switching elements in an upper arm and a lower arm, and converts direct current power. The failure detection portion detects a failure of an electric power converter or a failure of a winding group. The controller operates the switching elements and controls electricity supply. When the failure detection portion detects the failure, the controller stops output to an electric power converter in a failure system, and the controller reduces a total number of times of switching per unit time of the switching elements in a normally operating system.