The automatic transmission system includes a shift lever for changing stages by a user, a shift-lock device for fixing or releasing the shift lever, and a control unit including a shift control logic changing the stages of the transmission in accordance with the stage changed by the shift lever and a shift-lock control logic controlling the shift-lock device by stages for an initial time and a maintenance time after the initial time from when the stages may be changed such that the shift lever may be fixed, in order to shift-lock the shifted shift lever when the stages may be changed by the shift control logic.