In an automatic transmission with a speed changer having a plurality of engagement elements and configured to shift the speed changer into a selected one of a plurality of shift stages by switching engaged and disengaged states of the engagement elements, a first rotation sensor is provided for detecting input rotation of the speed changer and a second rotation sensor for detecting output rotation of the speed changer. Also provided is a transmission controller configured to determine whether a change in input torque inputted into the speed changer occurs. The transmission controller is further configured to determine that interlock has occurred in the speed changer, when there is no pulse signal output from the second rotation sensor though, during a vehicle stopped state, the input-torque change has occurred and the pulse signal from the first rotation sensor has been outputted.