To provide a controller for a rotary machine which can simplify a configuration required for checking the shutoff function of the switching device, and an electric power steering apparatus therewith. A controller for a rotary machine performs a high potential side forcible shutoff which forcibly shuts off a high potential side switching device of a diagnosis object phase in a drive state where the winding current of the diagnosis object phase becomes positive, or a low potential side forcible shutoff which forcibly shuts off a low potential side switching device of the diagnosis object phase in a drive state where the winding current of the diagnosis object phase becomes negative; and determines failure of the device shutoff unit based on a detection value of current or voltage when performing the high potential side forcible shutoff or the low potential side forcible shutoff.