The present disclosure relates to a technology which effectively absorbs a gear shifting impact by sequential fastening discs or plates to one another by a friction element. The friction element includes: a multi-plate disc set having discs and/or plates, and a fastening device having springs. In particular, the springs may start to be compressed from a spring having the smallest spring constant to a spring having a greater spring constant so as to sequentially fasten the discs and plates to one another so that the amount of a change in a stroke of a piston with respect to a change in an operation force of the piston is sequentially reduced.