In the Japanese Specifications for Highway Bridges, shock absorbing rubbers should be required to reduce the impact load with the collision between a superstructure and a device which prevents a superstructure from falling off, or between two superstructures. However, the specification has no detail prescript regarding the thickness of shock absorbing rubbers. Therefore, it is very important to investigate the effect of the thickness of shock absorbing rubbers on the reduction of impact load. In this study, the collision test between two steel solid bars using a new experimental apparatus is carried out in order to assess the above effect. This new apparatus can levitate a steel bar by using compressed air so that there is little friction between a steel bar and a guide rail against the horizontal movement. We name this apparatus as “frictionless impact testing apparatus”. The steel bar has 1000mm long, 200mm wide, 200mm high and about 3.0kN weight and the thickness of shock absorbing rubbers ranges from 2mm to 40mm and the collision velocity ranges from 0.2m/sec to 1.0m/sec on 7 stages. From the test results, it is confirmed that the sliding friction force is very small. In addition, the qualitative influence of the rubber thickness and collision velocity on the reduction of the maximum impact load is clarified in collision between two steel bars with shock absorbing rubber for bridge restrainer system.