The present invention provides an automatic transmission capable of properly judging faults of a switching mechanism. A control part (ECU) of the automatic transmission TM has an actual change gear ratio calculating part 1(0 )and a fault judging part (11). Under a condition that the control part ECU has recognized that the first brake device (B1) is switched to a reverse rotation preventing state, if the actual change gear ratio is kept to be an actual change gear ratio prior to a reduce of a rotational speed of a drive source (ENG) when the rotational speed of the drive source (ENG) is reduced, then the fault judging part (11) judges that a first brake (B1) has a fault.