Disclosed herein is a shock absorber for damping vibration transmitted to a vehicle according to a road state. The shock absorber includes a cylinder and a piston valve disposed within the cylinder and connected to a piston rod to divide the cylinder into a first chamber and a second chamber. The shock absorber further includes a hollow chamber formed within the piston rod, a floating piston disposed within the hollow chamber to move up and down and to divide the hollow chamber into upper and lower chambers, first and second orifices formed in upper and lower ends of the hollow chamber and connected to the first and second chambers, respectively, and a resistance changing mechanism configured to cooperate with the floating piston and change a fluid resistance with respect to the floating piston where the resistance changing mechanism cooperates therewith.