A gear support device of an automatic transmission reduces vibration and noise that are generated in a planetary gear set when power is transmitted from a vehicle driving source. In particular, the gear support device includes a gear support which is disposed on an outer peripheral surface of a hub retainer coupled to a planetary gear set directly connected to an input shaft of a transmission. The gear support rotatably supports the hub retainer by contacting with the outer peripheral surface of the hub retainer at every predetermined interval in a circumferential direction of the hub retainer.