  Based on a thoeretical model in which scalar fields play crucial roles, we
propose a mechanism to better understand a cosmological constant expected to be
small (nearly comparable with the critical density) but nonzero as suggested
strongly by the recent observations. We emphasize that a step further is needed
beyond the simplest scenario of a decaying cosmological constant, according to
which the effective $\Lambda$ is not a true constant but decays like $t^{-2}$,
thus explaining why any effect of $\Lambda$ which is assumed to be of the
Planckian order of magnitude in the earliest Universe is at most 120 orders
smaller today ($t\sim 10^{60}$ in units of the Planck time), without any
extreme fine-tuning of the parameters. In addition to the overall smooth
fall-off $\sim t^{-2}$, the total energy density, the sum of the contributions
from the ordinary matter and the scalar fields, must show some leveling-off
behavior, hence mimicking a truly constant $\Lambda$ to fit the observations.
We find that the desired behavior can be obtained by introducing two scalar
fields interacting with each other in a specific manner. Many aspects of this
model are discussed in detail, including the real origin of the whole
mechanism, and how the parameters and the initial conditions of the theory can
be determined by the cosomological parameters at the present epoch as well as
the past history of the Universe.
