Time-Of-flight Impact-Collision Ion Scattering Spectroscopy (TOF-ICISS) using 2 keV He$^$+/ ion was applied to study the geometrical structure of the Si(100) surface. The scattered ion intensity was measured along the [011] azimuth varying the incident angle. The focusing effects were appeared at the incident angles of 20$^{circ}$, 28$^{circ}$, 46$^{circ}$, 63$^{circ}$, and 80$^{circ}$. The Si atomic position was simulated by calculating the shadow cone to explain the five focusing effects. The four focusing effects at 28$^{circ}$, 46$^{circ}$, 63$^{circ}$, and 80$^{circ}$ resulted from the {011} surface where no dimers existed on the outermost surface. On the contrary, the scattering between two Si atoms in a dimer resulted in the focusing peak at 20$^{circ}$.