A control device for an automatic transmission capable of operating in an automatic shift mode in which a gear ratio is automatically selected based on driving conditions of a vehicle, and also capable of operating in a manual shift mode in which the gear ratio is changed based on an upshift command or a downshift command by manual operation of a manual operation device, the control device includes a manual shift control unit that changes and sets the gear ratio after downshift depending on road conditions when the downshift command has been operated by the manual operation device in the manual shift mode, and performs the downshift to the gear ratio that has been changed and set.