A device for controlling an automatic transmission including a lock-up clutch control portion and a zero slip control portion for bringing a lock-up clutch into a zero slip state immediately before slippage occurs in accordance with a zero slip request outputted during a non-gear shift, wherein in a case where a target slip amount is equal to or smaller than a slip amount threshold value upon transition to the zero slip state, the zero slip control portion fixes the target slip amount to the slip amount threshold value and retains the fixed target slip amount for a predetermined period of time, and after the predetermined period of time has elapsed, gradually decreases the target slip amount from the slip amount threshold value to a zero slip amount with a predetermined gradient with time.