The surface structure of a Si (100) substrate exposed to an As ionized cluster beam (ICB) was investigated by means of coaxial impact collision ion scattering spectroscopy (CAICISS). As a result of the measurements, first, it was proven that the Si surface is not damaged by As ICB exposure, although implanted As atoms of several percent were detected in the surface region. Second, both As and Si atoms from double domains of 2×1 and 1×2, and the predominant domain seems to be 2×1. Furthermore, 1×2 domain of As contributes to a lateral contraction of the surface.