An automatic transmission comprising: an input shaft that is rotatably installed on a fixing shaft 500; a hub housing into which the input shaft is inserted; a first clutch, of which one side is installed on the input shaft and the other side is installed at the hub housing; and a planetary gear portion comprising a carrier having a satellite gear that is engaged between a ring gear formed on inner walls of the hub housing and a sun gear formed on the input shaft; characterized in that the automatic transmission further comprises: a second clutch, of which one side is installed on the planetary gear portion and the other side is installed on the hub housing, wherein the first clutch and the second clutch include one-direction clutches that are locked in opposite directions, and wherein, when the input shaft is rotated in a forward direction, the first clutch is locked, and the second clutch is released so that the hub housing is output in the forward direction, and when the input shaft is rotated in the reverse direction, the first clutch is released, and the second clutch is locked so that the ring gear is rotated in the forward direction due to the satellite gear of the planetary gear portion that is rotated in the forward direction and a rotating speed of the hub housing is reduced and the hub housing is output in the forward direction, and when the hub housing is rotated in the forward direction, the second clutch is released, and a locking gap of the first clutch is formed to be greater than a locking gap of the second clutch, and when the hub housing is rotated in the reverse direction, the sun gear is rotated in the forward direction, the second clutch is first released before the first clutch is locked, so that the hub housing is rotated in the reverse direction and is capable of moving backward. an input shaft that is rotatably installed on a fixing shaft 500; a hub housing into which the input shaft is inserted; a first clutch, of which one side is installed on the input shaft and the other side is installed at the hub housing; and a planetary gear portion comprising a carrier having a satellite gear that is engaged between a ring gear formed on inner walls of the hub housing and a sun gear formed on the input shaft; characterized in that the automatic transmission further comprises: a second clutch, of which one side is installed on the planetary gear portion and the other side is installed on the hub housing, wherein the first clutch and the second clutch include one-direction clutches that are locked in opposite directions, and wherein, when the input shaft is rotated in a forward direction, the first clutch is locked, and the second clutch is released so that the hub housing is output in the forward direction, and when the input shaft is rotated in the reverse direction, the first clutch is released, and the second clutch is locked so that the ring gear is rotated in the forward direction due to the satellite gear of the planetary gear portion that is rotated in the forward direction and a rotating speed of the hub housing is reduced and the hub housing is output in the forward direction, and when the hub housing is rotated in the forward direction, the second clutch is released, and a locking gap of the first clutch is formed to be greater than a locking gap of the second clutch, and when the hub housing is rotated in the reverse direction, the sun gear is rotated in the forward direction, the second clutch is first released before the first clutch is locked, so that the hub housing is rotated in the reverse direction and is capable of moving backward. a second clutch, of which one side is installed on the planetary gear portion and the other side is installed on the hub housing, wherein the first clutch and the second clutch include one-direction clutches that are locked in opposite directions, and wherein, when the input shaft is rotated in a forward direction, the first clutch is locked, and the second clutch is released so that the hub housing is output in the forward direction, and when the input shaft is rotated in the reverse direction, the first clutch is released, and the second clutch is locked so that the ring gear is rotated in the forward direction due to the satellite gear of the planetary gear portion that is rotated in the forward direction and a rotating speed of the hub housing is reduced and the hub housing is output in the forward direction, and when the hub housing is rotated in the forward direction, the second clutch is released, and a locking gap of the first clutch is formed to be greater than a locking gap of the second clutch, and when the hub housing is rotated in the reverse direction, the sun gear is rotated in the forward direction, the second clutch is first released before the first clutch is locked, so that the hub housing is rotated in the reverse direction and is capable of moving backward.