Disclosed is a length-variable bumper capable of adjusting the length of a vehicle body to lengthen a time interval over which momentum changes upon collision. The length-variable bumper includes a bumper member primarily absorbing collision energy, a collision energy absorbing member foldable in a longitudinal direction, and an operation unit configured to transit the collision energy absorbing member to an unfolded state when a vehicle speed is a predetermined level or more and to transit the collision energy absorbing member to a folded state when the vehicle is the to predetermined level or less. The collision energy absorbing member has one end connected to a frame of the vehicle and the other end connected to the bumper member and configured to be deformed upon collision to secondarily absorb the collision energy.