  Superconductivity and polar charge fluctuation are studied in an organic
conductor with the dimer-molecule degree of freedom. The extended Hubbard
models, where the intra-dimer electronic structure and the inter-dimer Coulomb
interactions are taken into account, are analyzed by the random-phase
approximation and the fluctuation-exchange approximation. Superconductivity
appears in a vicinity of the charge-density wave (CDW) phase where the
electronic charge distributions are polarized inside of dimers. The extended
s-wave type paring is favored and its competitive relations with the
superconductivity due to the spin fluctuation depends on the triangular lattice
geometry. Comparison between two superconductivities realized near the polar
and non-polar CDW phases are also presented.
