An automatic transmission includes a main shaft, a counter shaft, a transmission mechanism capable of establishing a plurality of gear ranges by switching a transmission path of a driving force transmitted from the main shaft to the counter shaft by a combination of engagement and release of a plurality of engaging mechanisms, a determination unit configured to determine whether the transmission mechanism is switching the gear range, and a switching unit configured to switch information of a moment of inertia about the main shaft, which is used for control, between first information during establishment of the gear range and second information during switching of the gear range based on a determination result of the determination unit.