The adsorption mechanism and the origin of the In-induced surface electronic states of the Si(111)-(7x7) surface have been studied using bias-dependent scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). At very low In coverages, bias-dependent STM images show that some of adatoms have either slightly higher or lower apparent height in the filled-state image, while the empty-state images are completely identical to those of the clean Si(111)-(7x7). These peculiar features are interpreted as being caused by two adsorption mechanisms: charge redistribution due to the adsorption of In atoms on Si dangling bond and the substitution of In for Si atoms in the 7x7 adatom positions, opening the bandgap by removing intrinsic metallic surface states caused by Si adatom dangling bond. As the coverage increases, a self-assembly of a superlattice of two-dimensional non-metallic In nanodots on the Si(111)-(7x7) surface begin to develop on both halves of the 7x7 unit cell which indicates a metal-insulator transition occurring on the surface. This confirms that the origin of the metallicity of the Si(111)-(7x7) surface is the delicate charge transfer between adatom and restatom Si dangling bonds in the center of the 7x7 unit cell.