The irradiation of a surface with a cluster ion beam is known to give superior surface modification effects compared with monomer ions. Surface smoothing processes and sputtering yield are both better off. The irradiation process by cluster ions has proved to be quite different from that by monomer ions. We have examined the process of cluster impacts on a solid surface by molecular dynamics simulation to investigate these differences. We simulated an Ar cluster, with size from 13 to 3000 atoms, impacting on a Si(100) substrate, with acceleration energies from 0.5 keV to 55 keV. The Ar cluster atoms penetrate into the Si substrate keeping the cluster state, and the penetration depth is deeper than that of a monomer ion of the same velocity. Si atoms around the Ar cluster are displaced, some of them recover, and the Ar atoms of the cluster are reflected back into the vacuum. Consequently, the displaced Si atoms concentrate in a shallow surface area, and crater-like damage remains. The radius and the depth of the crater are almost the same and are proportional to the cube root of the acceleration energy of the cluster ion. This means that the peculiar damage formation process, created by a cluster ion impact, is caused by an isotropic energy transportation through many interactions between cluster and surface atoms.