A controversial issue of the driving force for the phase transition of the one-dimensional (1D) metallic In wires on Si(111) is studied by low-temperature scanning tunneling microscopy and spectroscopy. The energy gap opening and the longitudinal charge ordering through charge transfer at the Fermi level are unambiguously observed. The vacancy defects induce a local charge ordering decoupled from a lattice distortion above T(c), and pin the phase of charge order below T(c). All these results below and above T(c) including the detailed features such as local fluctuations strongly support the 1D charge-density-wave mechanism for the phase transition.