  Spin and charge fluctuations at vicinity of metal-to-Mott insulator
transitions are studied in an organic solid with molecular dimers. The extended
Hubbard model taking account of the internal electronic degree of freedom in a
molecular dimer is analyzed using the variational Monte Carlo method. Three
kinds of the electronic phases, i.e. a metallic phase, an antiferromagnetic
insulating phase and a polar charge ordered phase, compete with each other in
the ground state. It is found that the polar-charge fluctuation is dominant in
a wide range of the molecular dimerization and Coulomb interaction amplitudes,
and is enhanced remarkably near the metal-insulator phase boundary, in which
the spin fluctuation is almost unchanged. Implication for the $\kappa$-type
BEDT-TTF salts is discussed.
