  A phase diagram is drawn in a parameter space of the nearly half-filled
single band two-dimensional Hubbard model with U/t, t'/t and n as the
parameters, U, t, t' and n being the on-site interaction, the nearest and
second nearest neighbor transfer matrices and the number of electrons per site,
respectively. The parameter space is chosen so as to include the models for the
high-T_c cuprates as well as 2D-organic superconductors. The superconducting
and antiferromagnetic instability surfaces are drawn from the results of
calculations by using the spin fluctuation theory within the fluctuation
exchange approximation. The metal-insulator transition surface is also
indicated in the diagram. From this phase diagram we discuss the possibility of
the spin fluctuation mechanism being the common origin of the superconductivity
in the high-T_c cuprates and 2D-organic compounds.
