There is provided an automatic transmission capable of preventing deterioration of driving performance even when an engagement mechanism is not switched due to a failure. An automatic transmission includes a first brake which is always in a fixed state from a first forward gear stage to a predetermined gear stage and is in an open state at a gear stage with a higher speed than the predetermined gear stage, and a hydraulic sensor configured to detect whether the first brake is switched to the fixed state. When a transmission control device instructs the first brake to switch to the fixed state, and it is confirmed that the first brake is in an open state, the transmission control device instructs engagement mechanisms other than the first brake to perform switching so that a preliminary gear stage is able to be set.