To provide an automatic transmission having an electrically driven throttle valve, a clutch actuator and a shift actuator in which shift shock and degradation of riding feeling caused by a failure of the clutch actuator are prevented.

A controller 90 of an automatic transmission comprises a clutch signal supplying unit 92 for supplying a driving signal to a clutch actuator 60, a clutch position detecting unit 94 for detecting a position of a clutch 44, a shift signal supplying unit 97 for supplying a driving signal to a shift actuator 70, a determination unit 96 for acquiring the position of the clutch 44 when a prescribed time has passed after the clutch signal supplying unit 92 supplied the driving signal and determining the position of the clutch 44, and a shift signal prohibition unit 98 for prohibiting supply of the driving signal from the shift signal supplying unit 97 if the determination unit 96 determines that the position of the clutch 44 is equal to or less than a threshold.