A central air bubble is observed to be maintained during the process of an elastic hemi-sphere impacting on a smooth rigid surface. The similar phenomenon has been found in droplet impact in other studies. The effect of air gap between the two solids has been always neglected in the previous studies of the impact problem. We find that the central air bubble has a big contribution to the energy loss in the second phase of impact process. Its negative pressure under the tensile force prevents the hemi-sphere impactor from rebounding from the surface. The impactor made of softer material or with a lower impact velocity often can lead to a larger initial size of central air bubble. But the high impact velocity can increase the negative pressure and the resistance brought by it. With high impact velocities and softer material, the propagation of outward contact can also become faster thanks to the decline of air defects and the disappearance of instability patterns. We guess that the air permeates into the tiny cracks on the impactor surface, which appear due to the high stress.