An impact absorbing device for a vehicle may include a bumper beam having a hollow cross section which extends in a traverse direction of the vehicle, and a crash box interposed between the bumper beam and a vehicle body in a longitudinal direction of the vehicle at both distal portions of the bumper beam, wherein rear surfaces of the both distal portions in the bumper beam include an opening formed so as to communicate with the hollow cross section of the bumper beam, wherein a rib may be formed at the opening of the bumper beam in a traverse direction of the opening, and wherein the crash box includes a front tip end inserted through the opening while being fitted to the rib to extend up to an inside of the hollow cross section, and a rear tip end mounted to the vehicle body in the longitudinal direction of the vehicle.