A control device of an automatic transmission controls an automatic transmission 1 comprising a transmission mechanism 3 including a plurality of engagement elements, and a hydraulic oil supply device 4 supplying hydraulic oil to the transmission mechanism The control device of the automatic transmission comprises an engagement element control part 41 configured to use the hydraulic oil supply device to make the plurality of engagement elements change between an engaged state and a disengaged state; and a deceleration degree calculating part 42 configured to calculate a target deceleration degree of a vehicle in which the automatic transmission is provided. The engagement element control part is configured to make the engagement element in the disengaged state engage so that the vehicle decelerates if the target deceleration degree is equal to or more than a predetermined value when an increase in temperature of hydraulic oil in the automatic transmission is demanded.