The present invention relates to a range switching device for performing range switching of an automatic transmission by using an actuator that is electrically controlled to be driven, and to a switching method thereof. A range switching device of an automatic transmission that can reduce collision noise occurring from a detent mechanism and a switching method thereof are provided. The automatic transmission includes a detent mechanism that presses a roller against a groove by using a spring, moves the roller between grooves, and determines a range position of the transmission, and a range of the transmission is changed by driving the detent mechanism by the actuator. In changing a range position of the transmission, the actuator is braked in accordance with a drawing force of the spring into a groove so that energy of the velocity when the roller reaches a target stop position can be controlled to approach zero.