out the first planetary gear device pg 1 the second planetary gear device pg 2 the third planetary gear device pg 3 and the fourth planetary gear device pg 4 may be sequentially arranged along the axial direction of the input shaft in and the output shaft out more specifically the first rotary element s 1 of the first planetary gear device pg 1 may be installed to be fixable to a transmission case cs by means of a fourth clutch cl 4 from among the at least six shifting elements the first rotary element s 2 of the second planetary gear device pg 2 may be installed to be fixable to the transmission case cs by means of a third clutch cl 3 from among the at least six shifting elements the first rotary element s 4 of the fourth planetary gear device pg 4 may be installed to be fixable to the transmission case cs by means of a second clutch cl 2 from among the at least six shifting elements accordingly the second clutch cl 2 the third clutch cl 3 and the fourth clutch cl 4 function as brakes respectively such that the first rotary element s 4 of the fourth planetary gear device pg 4 the first rotary element s 2 of the second planetary gear device pg 2 and the first rotary element s 1 of the first planetary gear device pg 1 may be converted to a rotatable state wherein rotation is allowed or a restrained state wherein rotation is restricted by means of the operations of the second clutch cl 2.